Fix ~=X.Y.Z requires-python inferring later Python versions - #5578

Description

Fixes #5468.

strip_specifier_set turned ~=3.11.0 into ~=3.11. That drops the upper bound: ~=3.11.0 means >=3.11.0, ==3.11.* (only 3.11), while ~=3.11 means >=3.11, ==3.*. So requires-python = "~=3.11.0" inferred py311 through py315. A compatible-release specifier with a patch component (three or more release parts) is now stripped to ==X.Y.*; the two-part form is unchanged.

Stability. The minimum inferred target version never rises. For ~=3.7.x through ~=3.15.x the feature set of the inferred targets is the same on main and with this change (checked for every minor version, with and without a patch number), so stable formatting does not change. Only ~=3.3.x to ~=3.6.x differ, and there the new result is the correct one. In a sweep of 6,084 single and pairwise specifier sets, the 445 results that changed all come from a set containing a ~= with three or more release parts.

Known edge, not changed here. ~=3.11.4,!=3.11.0 and ~=3.11.0,>3.11 now give None (on main they gave py312 and up; the right answer is py311). That comes from the existing handling of !=X.Y.0 and of a two-part > (#3581), not from this change.

An earlier pull request for this issue, #5469, took the same approach and was closed by its author without a review. This one was made independently, as described below.

How this was made (AI disclosure). FixedYet, a tool that reproduces bug reports in a sandbox, wrote a failing test for the issue on main; NVIDIA Nemotron, through FixedYet, proposed the change to strip_specifier_set, and FixedYet verified it (the test fails without it and passes with it, and the suite had no new failures). A review with Claude (Anthropic) checked it against PEP 440, moved it to the literal ==X.Y.* form with a comment, and added the table cases and the changelog entry. I read the final diff myself before marking this ready for review.

Checked locally with Black's CI commands, in a clean Python 3.12 container:

• pytest tests --run-optional no_jupyter --numprocesses auto: 596 passed, 1 skipped (IPython is optional), the same as on main
• pytest tests --run-optional jupyter -m jupyter --numprocesses auto: 83 passed
• black, flake8, isort and mypy (with the pre-commit hook's dependencies) on the changed files, and prettier on CHANGES.md: clean
• the new table cases fail on main without the change

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? (not a style change; see Stability above)
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation? (none needed)