Add pytest.ini to register custom pytest marks

[cderici]

Description

Add a .pytest.ini at the top level to register custom pytest marks to prevent pytest from emitting warnings for each mark, such as:

tests/unit/test_model.py:371: 24 warnings
  /home/caner/work/python-libjuju/tests/unit/test_model.py:371: PytestUnknownMarkWarning: Unknown pytest.mark.wait_for_idle - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    @pytest.mark.wait_for_idle

QA Steps

No QA needed.

Needs to be forward-ported to the main branch. Proactively added the bundle mark as it's used in there.

[cderici]

/merge