Update python Docker tag to v3.14

[renovate]

This PR contains the following updates:

Package	Update	Change
python	minor	3.11 → 3.14

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[chrishayuk]

Not merging: this is the exact .python-version bump that caused main to be broken for 3 months (fixed in #38) — bumping it again reintroduces the risk.

More importantly, I verified .python-version silently overrides whatever Python version uv python install X explicitly installs in a CI job. Since test.yml's matrix jobs run uv python install ${{ matrix.python-version }} followed by a bare uv sync --dev (no --python flag), merging this would make every matrix entry (3.11, 3.12, 3.13) silently run under 3.14 instead of their declared version — defeating the whole point of the test matrix.

I also tested 3.14 directly: there's a real test failure (test_cleanup_import_error, likely a stdlib unittest.mock/import-machinery behavior change) and an mypy failure (numpy's bundled 3.14-targeting stubs use type alias syntax requiring python_version >= 3.12, conflicting with this project's python_version = "3.11" mypy config). Worth revisiting once 3.14 is added to the actual test matrix and both issues are diagnosed properly, but not as a routine Renovate bump.

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