uv-pep440: DRY up VersionSmall implementation (#8834)
This PR simplifies the VersionSmall implementation a bit by utilizing
more constants. That is, if the bit-level format changes, *most* of
those changes should be implementable by just changing the constants.
Previously, you would need to audit and tweak the code as well. (The
exception here is `push_release`. If the release segment bit format is
changed, then that function will need to be tweaked. I didn't think it
was worth over-complicating things to make its implementation more
general.)
