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

[bhrutledge]

@PureTryOut Thanks for the suggestion, but we're relying on new behavior in importlib_metadata, which will eventually be merged into Python's importlib.metadata. See #732 for details, including a discussion with a Fedora maintainer about the implications: #732 (comment).

[PureTryOut]

Ouch, that I did not realize. The explanation by @jaraco there is sane and reasonable, I'll go ahead and just repackage importlib_metadata on Alpine Linux (for which I mainly made this MR) then.

Closing this and jaraco/keyring#507