Update CI actions

[petercorke]

• update Python used for sphinx build, coverage, testing
• update version of actions used.

CI: update workflow actions and fix test environment

• actions/checkout@v2 → @v4 in all three workflows — v2 is deprecated and has known security and compatibility issues
• MPLBACKEND: TkAgg → Agg in the test job — TkAgg requires a display server and is unreliable on headless runners; Agg is the correct non-interactive backend for CI
• Python 3.8 → 3.12 in the codecov job and publish.yml — Python 3.8 reached end-of-life in October 2024
• To make this work, I had to do another round of changes. The Agg backend (for headless graphics) was hanging in the plt.pause() so added logic to skip certain unit tests in the CI environment.

Remaining known issues (tracked separately):

• codecov/codecov-action@v3 should be updated to @v4
• ad-m/github-push-action@master in sphinx.yml should be pinned to a specific commit SHA rather than a branch to prevent supply-chain risk
• ubuntu-22.04 is hardcoded; consider ubuntu-latest
• The windows-latest exclusions for Python 3.11 and 3.12 lack a comment explaining the reason

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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