Use pathlib instead of py

[Avasam]

Summary of changes

py was only installed as a dependency of pytest-fork which was only installed as a dependency of pytest-xdist. This also removes another untyped import for #2345 / #3979 / #4192

I could've bumped it to >=1.9.0 (first typed version: https://py.readthedocs.io/en/latest/changelog.html#id3) but I figured may as well just stop using it since: https://pypi.org/project/py/

NOTE: this library is in maintenance mode and should not be used in new code.
The py lib is a Python development support library featuring the following tools and modules:
py.path: uniform local and svn path objects -> please use pathlib/pathlib2 instead

Pull Request Checklist

• Changes have tests (the changed tests themselves)
• News fragment added in newsfragments/.
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