- Use pytest>=6.2 features (same as install_requires).
When we can require >=7, can fix some more typing omissions and
version checks.
- Replace testdir with pytester
- Replace py.test with pytest
- Replace tmpdir with tmp_path
- Remove (almost) all other uses of py
- Add some type annotations (not checked yet)
Ref #722.
This makes things much easier in terms of typing.
pytest 6.2 has been out for a ~year so if someone updates pytest-xdist
they should also update pytest!
Fix#720.
