Update test suite to modern pytest

- 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.

tox.ini

@@ -18,7 +18,6 @@ commands=
 extras =
   testing
   psutil
-deps = pytest
 commands =
   pytest {posargs:-k psutil}