Fix regression and restore stdlib reader suppression on Python 3.10

[jaraco]

• Add test capturing missed expectation. Ref files(...).joinpath doesn't accept variable number of arguments in Python 3.10+ #257.
• Mark test as xfail on Python 3.10 where it fails. Ref files(...).joinpath doesn't accept variable number of arguments in Python 3.10+ #257.
• Ensure stdlib readers are excluded on Python 3.10 even when found in importlib.readers. Closes files(...).joinpath doesn't accept variable number of arguments in Python 3.10+ #257.
• Add news fragment.