Make untagged versions PEP 440-compliant

[savannahostrowski]

Related to python/cpython#99968

[ned-deily]

While making this change, it would be good to also address the updating of the README file (#169).

[hugovk]

In python/cpython#99968 (comment) you suggested targeting at least 3.16 onward, although this PR applies to all of 3.10-3.16.

If we only target 3.16, this won't have any affect until after the first 3.16 alpha in October (it's on 3.16.0a0 until then). I think it's okay to also apply this to the 3.15 beta, to help shake out any issues.

Shall we do that for a few 3.15 releases before applying it to the full set, or just go for them all now?

[savannahostrowski]

@hugovk If you're cool with that, I think 3.15+ would be great. I've pushed a change for that.

@ned-deily I think I'd prefer to tackle this in a separate PR so we can keep this one scoped on the PEP 440 compliance. I've assigned myself to #169 though, so I can look at that next.