community/py3-twine: re-enable tests and add missing importlib-metadata dep

CC @Leo, so it seems we can't do without py3-importlib-metadata after all.

It seems that newer versions of this package still contains functionality not present in the Python standard library at least until Python 3.10, and some packages (py3-keyring, py3-twine) depend on that.

Read https://github.com/pypa/twine/pull/732#issuecomment-809755307 for context.