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.

src/xdist/workermanage.py

@@ -423,9 +423,9 @@ def unserialize_warning_message(data):
 
     kwargs = {"message": message, "category": category}
     # access private _WARNING_DETAILS because the attributes vary between Python versions
-    for attr_name in warnings.WarningMessage._WARNING_DETAILS:
+    for attr_name in warnings.WarningMessage._WARNING_DETAILS:  # type: ignore[attr-defined]
         if attr_name in ("message", "category"):
             continue
         kwargs[attr_name] = data[attr_name]
 
-    return warnings.WarningMessage(**kwargs)
+    return warnings.WarningMessage(**kwargs)  # type: ignore[arg-type]