Classes missing in documentation of MorphoCut

Describe the bug

Project respository: https://github.com/morphocut/morphocut/

When I build the documentation with 3.3, many classes are missing. Not error is shown on the console.

Issue at RTD: https://github.com/readthedocs/readthedocs.org/issues/7703

To Reproduce
Steps to reproduce the behavior:

$ git clone https://github.com/morphocut/morphocut/
$ cd morphocut
$ pip install -e .[docs]
$ cd docs
$ make html
$ # open _build/html/stream.html and see that it does not contain any classes.

Expected behavior
The page should contain the classes TQDM, Slice, ...

Your project
https://github.com/morphocut/morphocut/

Environment info

- OS: Ubuntu
- Python version: 3.7.9
- Sphinx version: 3.3.1
- Sphinx extensions: [
    "sphinx.ext.autodoc",
    "sphinx.ext.coverage",
    "sphinx.ext.napoleon",
    "sphinxcontrib.programoutput",
    "sphinx.ext.viewcode",
    "sphinx.ext.intersphinx",
    "sphinx_autodoc_typehints",
    "sphinx.ext.coverage",
    "sphinx.ext.autosummary",
]

Additional context

It seems that exactly these classes are missing that are decorated with @ReturnOutputs.

Reproduce the issue by following the documented clone, install, and make html steps, then inspect docs/_build/html/stream.html. Start with the Sphinx autodoc and autosummary extensions and the @ReturnOutputs decorator context; done means the generated page includes classes such as TQDM and Slice without console errors.