Support Python 3.15 and refresh dev tooling

[eriknw]

No library changes were needed for 3.15; the full suite, doctests and a wheel/sdist build all pass on 3.15.0rc2. Just declare support and update the tooling around it.

• Declare 3.15: classifier, tox envlist, and CI matrix (3.15 and 3.15t, with allow-prereleases since 3.15 is still at rc)
• Bump GitHub Actions and pre-commit hooks to current releases, and hash-pin the remaining floating action refs, which zizmor 1.30 requires
• Add a PEP 735 test dependency group, used by both CI and tox
• Fix codespell's file filter: it pointed at docs/, so doc/ was never checked
• Drop a stale duplicate PYPY definition in functoolz that the later definition already shadowed
• readthedocs: ubuntu-24.04 and Python 3.14; re-enable fail_on_warning now that the _static warning is gone
• dependabot: 7-day cooldown and grouped updates
• Build wheels on 3.14 instead of 3.9, and fix the workflow name typo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a1e25cb) to head (dfbdee7).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #636   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1226      1225    -1     
=========================================
- Hits          1226      1225    -1     

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