CI: Update runners, Python matrix and Github Actions versions

[KevinRK29]

• Matrix: ubuntu-latest, CPython 3.11 + PyPy 3.8/3.10 (drop 3.8)
• actions/checkout@v4, actions/setup-python@v5
• MyPy step runs only when Python is not PyPy (startsWith guard)
• Minor mypy annotation fix for mypy 2.0
• Update stale coverage.yaml file

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?