RFE: Replace use of `pkg_resources` with `importlib.metadata`

[Tomasz Kłoczko]

See discussions:
astropy/astropy#11091
pypa/pip#7413

[tkloczko@devel-g2v SRPMS]$ rpm -ql python-py | grep py$ | xargs grep pkg_resources
grep: /usr/lib/python3.8/site-packages/py: Is a directory
/usr/lib/python3.8/site-packages/py/_vendored_packages/apipkg/__init__.py:    from pkg_resources import get_distribution, DistributionNotFound

[Ronny Pfannschmidt]

Unplanned, this library will be discarded soon as pytest sheds the last usages

See #287 and #288