Needs a backport package until we no longer support Python 3.7; stdlib
gained `importlib.metadata` in Python 3.8.

I should note that installing with `-r dev-requirements.txt` will always
(currently) install the `importlib_metadata` backport because of Sphinx.

Unfortunately this one is *not* yet part of the stdlib in any Python
release, but there's no point in removing only *most* uses of the older
`pkg_resources` library. Gotta do them *all*.

Adapted the `get_meta_description()` for `EntryPointPlugin` to return
the expected format. Could alternatively change the expectation of
`test_get_label_entrypoint()`.

As the Python packaging docs say, entry points are now a PyPA-defined
interoperability specification not confined to packages built using
`setuptools`, and we should update our docs to reflect that.

See https://packaging.python.org/en/latest/specifications/entry-points/

It's not a "setuptools entry point"; it's an "entry point group".