Someday: Use `importlib.metadata` from the standard library

[bhrutledge]

Follow-up to #772 (review) and #732 (comment).

It'd be nice to remove the dependency on importlib_metadata in Python 3.10+, but Twine will still need it for as long as it supports older versions of Python. So, I think it's worth weighing the complexity of conditional imports vs. an additional dependency.

I think this should at least wait until 3.10 is officially released and is added to Twine's testing matrix. If we do decide to use importlib.metadata, then I think the version check should be done in one place, as suggested in #772 (comment).

[jakkdl]

3.10 is now officially released, so this could be revisited. I'm open for writing a PR if you want.