Get version with importlib.metadata

[KRunchPL]

pkg_resources that was used to get package version is a deprecated API: https://setuptools.pypa.io/en/latest/pkg_resources.html

[regebro]

Thanks!

If I understand correctly, we don't need the backport on Python 3.8, when importlib.metadata is in Python? In that case, let's hold off until we drop support for 3.7, otherwise, we'll just have another bit of old code to remove later, which means we didn't really gain anything.

[regebro]

OK, implemented, thanks!