Only use external importlib_metadata package when python < 3.8

[PureTryOut]

importlib_metadata has been integrated into the Python standard library
in 3.8 and is thus unnecessary for any newer versions.
Let's only use the external importlib_metadata package when actually
running on a Python version that doesn't include it already

This is mostly useful for distributions where there is no need to
package importlib_metadata anymore, and it's future proofing for when
the external package stops being developed anymore

[PureTryOut]

I... Don't understand why tests are failing. Locally they succeed fine, and I compared the output of the failing matches variable on both Python 3.7.10 and on 3.9.4, both were the same. I'm not sure why Python behaves differently on CI.

[PureTryOut]

Never mind, closing this after reading pypa/twine#732 (comment)

Too bad