5.13.0: pytest is failing with error in tests/test_typing.py

I'm packaging your module as an rpm package so I'm using the typical PEP517 based build, install and test cycle used on building packages from non-root account.
- `python3 -sBm build -w --no-isolation`
- because I'm calling `build` with `--no-isolation` I'm using during all processes only locally installed modules
- install .whl file in using 'installer` module
- run pytest with $PYTHONPATH pointing to sitearch and sitelib inside
- build is performed in env which is *`cut off from access to the public network`* (pytest is executed with `-m "not network"`)

Here is pytest output:

```console
+ PYTHONPATH=/home/tkloczko/rpmbuild/BUILDROOT/python-traitlets-5.13.0-2.fc35.x86_64/usr/lib64/python3.8/site-packages:/home/tkloczko/rpmbuild/BUILDROOT/python-traitlets-5.13.0-2.fc35.x86_64/usr/lib/python3.8/site-packages
+ /usr/bin/pytest -ra -m 'not network'
============================= test session starts ==============================
platform linux -- Python 3.8.18, pytest-7.4.3, pluggy-1.3.0
rootdir: /home/tkloczko/rpmbuild/BUILD/traitlets-5.13.0
configfile: pyproject.toml
testpaths: tests, examples
collected 569 items / 2 errors

==================================== ERRORS ====================================
____________________ ERROR collecting tests/test_typing.py _____________________
'mypy_testing' not found in `markers` configuration option
____________________ ERROR collecting tests/test_typing.py _____________________
'mypy_testing' not found in `markers` configuration option
=========================== short test summary info ============================
ERROR tests/test_typing.py - Failed: 'mypy_testing' not found in `markers` configuration option
ERROR tests/test_typing.py - Failed: 'mypy_testing' not found in `markers` configuration option
!!!!!!!!!!!!!!!!!!! Interrupted: 2 errors during collection !!!!!!!!!!!!!!!!!!!!
============================== 2 errors in 0.77s ===============================
```

Here is list of installed modules in build env

```console
Package Version
----------------------------- ---------
alabaster 0.7.13
argcomplete 3.1.4
Babel 2.13.1
build 1.0.3
charset-normalizer 3.3.2
cppclean 0.13
decorator 5.1.1
distro 1.8.0
docutils 0.20.1
editables 0.5
exceptiongroup 1.1.3
gpg 1.23.0
hatchling 1.18.0
idna 3.4
imagesize 1.4.1
importlib-metadata 6.8.0
iniconfig 2.0.0
installer 0.7.0
Jinja2 3.1.2
markdown-it-py 3.0.0
MarkupSafe 2.1.3
mdit-py-plugins 0.4.0
mdurl 0.1.2
myst-parser 2.0.0
packaging 23.2
pathspec 0.11.2
pluggy 1.3.0
Pygments 2.16.1
pyproject_hooks 1.0.0
pytest 7.4.3
python-dateutil 2.8.2
pytz 2023.3
PyYAML 6.0.1
requests 2.31.0
six 1.16.0
snowballstemmer 2.2.0
Sphinx 7.1.2
sphinx-rtd-theme 1.2.2
sphinxcontrib-applehelp 1.0.4
sphinxcontrib-devhelp 1.0.5
sphinxcontrib-htmlhelp 2.0.4
sphinxcontrib-jquery 4.1
sphinxcontrib-jsmath 1.0.1
sphinxcontrib-qthelp 1.0.3
sphinxcontrib-serializinghtml 1.1.9
tomli 2.0.1
trove-classifiers 2023.11.2
urllib3 1.26.18
wheel 0.41.3
```