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.

setup.cfg

@@ -53,7 +53,6 @@ pytest11 =
 [options.extras_require]
 testing =
     filelock
-    pytest
 psutil = psutil>=3.0
 setproctitle = setproctitle