Circular imports in builtin extensions Sphinx-4.0.1

Describe the bug
A project starts to fail building with with Sphinx-4.0.1 with ImportError(s) being raised during setup_extension in Sphinxs application constructor. Note that these extensions are builtin, so there seems nothing can be done on the user side to prevent this (right?).

# from sphinx/application.py:237

        # load all built-in extension modules
        for extension in builtin_extensions:
            self.setup_extension(extension)

Just booting Sphinx fails already, with this stacktrace:

runfile('sphinx', args=['-W', '-n', '-b', 'html', '-d', 'build/doctrees', 'doc', 'build/html', '--keep-going', '-v', '-D', 'nbsphinx_kernel_name=weldx', '-D', 'nbsphinx_execute=never'], wdir='/home/marscher/sources/weldx/', is_module=True)
Running Sphinx v4.0.1
Original exception:
Traceback (most recent call last):
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/registry.py", line 420, in load_extension
    mod = import_module(extname)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/epub3.py", line 18, in <module>
    from sphinx.builders import _epub_base
  File "/home/marscher/software/pycharm-2019.2.4/plugins/python/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/_epub_base.py", line 23, in <module>
    from sphinx.builders.html import BuildInfo, StandaloneHTMLBuilder
  File "/home/marscher/software/pycharm-2019.2.4/plugins/python/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/html/__init__.py", line 49, in <module>
    from sphinx.writers.html import HTMLTranslator, HTMLWriter
  File "/home/marscher/software/pycharm-2019.2.4/plugins/python/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/writers/html.py", line 32, in <module>
    from sphinx.builders.html import StandaloneHTMLBuilder
ImportError: cannot import name 'StandaloneHTMLBuilder' from partially initialized module 'sphinx.builders.html' (most likely due to a circular import) (/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/html/__init__.py)
Traceback (most recent call last):
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/registry.py", line 420, in load_extension
    mod = import_module(extname)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/epub3.py", line 18, in <module>
    from sphinx.builders import _epub_base
  File "/home/marscher/software/pycharm-2019.2.4/plugins/python/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/_epub_base.py", line 23, in <module>
    from sphinx.builders.html import BuildInfo, StandaloneHTMLBuilder
  File "/home/marscher/software/pycharm-2019.2.4/plugins/python/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/html/__init__.py", line 49, in <module>
    from sphinx.writers.html import HTMLTranslator, HTMLWriter
  File "/home/marscher/software/pycharm-2019.2.4/plugins/python/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
    module = self._system_import(name, *args, **kwargs)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/writers/html.py", line 32, in <module>
    from sphinx.builders.html import StandaloneHTMLBuilder
ImportError: cannot import name 'StandaloneHTMLBuilder' from partially initialized module 'sphinx.builders.html' (most likely due to a circular import) (/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/html/__init__.py)
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/cmd/build.py", line 281, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/application.py", line 239, in __init__
    self.setup_extension(extension)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/application.py", line 400, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/registry.py", line 423, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname,
sphinx.errors.ExtensionError: Could not import extension sphinx.builders.epub3 (exception: cannot import name 'StandaloneHTMLBuilder' from partially initialized module 'sphinx.builders.html' (most likely due to a circular import) (/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/html/__init__.py))
Extension error:
Could not import extension sphinx.builders.epub3 (exception: cannot import name 'StandaloneHTMLBuilder' from partially initialized module 'sphinx.builders.html' (most likely due to a circular import) (/home/marscher/miniconda3/envs/weldx/lib/python3.9/site-packages/sphinx/builders/html/__init__.py))
Process finished with exit code 2

To Reproduce
Steps to reproduce the behavior:

$ git clone https://github.com/BAMWelDX/weldx/
$ cd weldx
$ pip install -e .
$ sphinx-build -W -n -b html -d build/doctrees doc build/html --keep-going

Expected behavior
No circular imports during importing builtin extensions.

Your project
https://github.com/BAMWelDX/weldx/

Failing CI: https://github.com/BAMWelDX/weldx/pull/355/checks?check_run_id=2598128954
RTD: https://readthedocs.org/projects/weldx/builds/13779787/

Environment info

• OS: Linux
• Python version: 3.9.2
• Sphinx version: 4.0.2
• Sphinx extensions: recommonmark, sphinxcontrib.napoleon, nbsphinx, sphinx.ext.autodoc, sphinx.ext.autosummary, sphinx.ext.intersphinx, sphinx.ext.mathjax, sphinx_copybutton, sphinx_asdf, numpydoc, sphinx_autodoc_typehints

Workaround
Hack into sphinx/cmd/build.py:31

from sphinx.util.docutils import docutils_namespace, patch_docutils
# prevent circular imports...
import sphinx.builders.html
import sphinx.builders.latex
import sphinx.builders.texinfo
import sphinx.builders.text
import sphinx.ext.autodoc

def handle_exception(app: Sphinx, args: Any, exception: BaseException, stderr: IO = sys.stderr) -> None:  # NOQA

This makes the build passing again.

Start with the builtin extension loading in sphinx/application.py and the import paths shown through sphinx/builders/epub3.py, sphinx/builders/_epub_base.py, sphinx/builders/html/init.py, and sphinx/writers/html.py. Reproduce the failure with the listed sphinx-build command and inspect why the HTML builder and writer import each other during setup. Done means builtin extensions load without circular-import errors and the documented reproduction builds successfully.