tox: drop python3.8 backported deps
tox: drop python3.8 backported deps

importlib_metadata is backport of Python 3.8's standard library for
older Python.

We've upgraded to Python 3.8, drop it.

See: https://importlib-metadata.readthedocs.io/en/latest/

necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages