announce (and warn about) end of Python 2.7 support starting in v2.24.x

[tdyas]

Announce that v2.23.x is the last Pants release series to proactively test that it works with Python 2.7.

Also, log a warning when Python 2 is detected in the default interpreter constraints or in an interpreter_constraints field. The warning can be disabled in case the user really knows what they want via the new --no-python-warn-on-python2-usage option.

[huonw]

Thanks for taking this on

[tdyas]

Fixed what should be the final test failure at https://github.com/pantsbuild/pants/actions/runs/10785314963/job/29910366528?pr=21387#step:11:369 which is due to tests being skipped locally intentionally because not all Pythons were present on my system.

Not sure why those tests passed ever because the Address field used for the expected value in the comparisons was a dummy value which does not reflect the addresses of generated targets,.

[huonw]

Looks good except for getting @memoized working properly.

[huonw]

I think the memoized here won't do quite the right thing, as each call of warn_on_python2_usage_in_interpreter_constraints will create a new inner and thus a new batch of memoization.

I think the @memoized needs to be a on a top level function?

[tdyas]

You're right. The lists being passed in to function in an earlier iteration are not hashable, so my brain was trying an "easy" fix to consolidate turning them into tuples for @memoized to work. Will just reorganize them into two free functions so the @memorized is technically on an outer function.