ci(pins): make the python-classifier check advisory, not fatal

[lstein]

Summary

Follow-up to the review comment on #9351: the pin check treated project.classifiers as an authoritative allowlist, which is wrong — classifiers are optional in PEP 621 and purely informational on PyPI, so they cannot decide whether a pin is installable.

@JPPhoto's repro, on #9351 as it stands:

$ jq '.python = "3.11"' pins.json | sponge pins.json   # pyproject.toml untouched
$ python3 scripts/check_pins.py; echo "exit: $?"
pins.json is out of sync with pyproject.toml:
  - pins.json python is '3.11' but pyproject.toml's classifiers declare support only for 3.12; ...
exit: 1

requires-python = ">=3.11, <3.13" genuinely permits that pin. The metadata allows the install; only a piece of documentation disagreed.

After this PR:

$ python3 scripts/check_pins.py; echo "exit: $?"
warning: pins.json python is '3.11', which pyproject.toml's classifiers do not mention
(they list 3.12). That is allowed - classifiers are informational, and requires-python is
what gates installation - but consider adding 'Programming Language :: Python :: 3.11' so
the published metadata matches.
pins.json is consistent with pyproject.toml
exit: 0

What changed

• requires-python is once again the only authority the script fails on for the python pin.
• A pin the classifiers don't mention is a warning naming the classifier to add.
• Absent, malformed, or project.dynamic classifiers are no longer a finding at all — classifiers is optional, so its absence says nothing about the pin.
• The advisory still prints on a failing run: a wrong pin and an unmentioned pin are usually the same edit.

What this gives up, deliberately

A version that satisfies an open-ended requires-python but that no interpreter has — >=3.11 with a 3.99 pin — is no longer caught. The module docstring says so explicitly, and there's a test named after it.

Catching that case was the reason the classifier allowlist went in, but the price was rejecting legal pins, and a checker that blocks a correct change is worse than one that misses an implausible typo. The pin still has to be major.minor with no patch component, and it still has to satisfy requires-python — with the repo's current >=3.11, <3.13, there is no reachable gap.

Testing

116 tests in tests/test_check_pins.py, all green; ruff format/check clean; script verified under a bare python3 and parsed against the 3.11 grammar (CI runs it with whatever python3 the runner has).

New tests: a 3.11 pin passes with only an advisory; the advisory names the classifier to add; a pin the classifiers do mention says nothing; missing / malformed / dynamic classifiers are not findings; a 3.012 classifier does not count as declaring 3.12; and the advisory survives a failing run.

Mutation-tested the new logic — every mutant applied to the advisory path and the warning loop is caught, including the two that survived my first pass at this change.

[lstein]

#9351 merged while this was in review, so I rebased — the diff is now just this PR's two commits.