Support for pytest 7.0

[GergelyKalmar]

I'm getting the following error with pytest 7.0:

E   pytest.PytestRemovedIn8Warning: The (fspath: py.path.local) argument to MypyFile is deprecated. Please use the (path: pathlib.Path) argument instead.
E   See https://docs.pytest.org/en/latest/deprecations.html#fspath-argument-for-node-constructors-replaced-with-pathlib-path

It would be awesome if the plugin could be updated to continue working with pytest.

[jaraco]

See jaraco/pytest-checkdocs#9 and jaraco/pytest-perf#5 for approaches I used to fix this issue.