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

[kloczek]

See discussions:
astropy/astropy#11091
pypa/pip#7413
pypa/setuptools#3279

[tkloczko@devel-g2v youtube-1.2.0]$ grep -r pkg_resources
sphinxcontrib/__init__.py:__import__('pkg_resources').declare_namespace(__name__)
build/lib/sphinxcontrib/__init__.py:__import__('pkg_resources').declare_namespace(__name__)

[kloczek]

Thx 👍
I'll test that on next relese.