Address `ast` deprecation warnings

[edgarrmondragon]

Description

• Adapted code for future removal of ast.Num, ast.Str and ast.NameConstant
• Added tests for stable Python 3.12, alpha 3.13, as well as pypy 3.9 and 3.10
• Added continue-on-error: true for 2.7 cause it's no longer available with actions/setup-python

Pre-approval checklist (for submitter)

Please complete these steps

• [x] Passes tests
• [ ] New tests for additional features or changed functionality
• [x] My name and contribution added to contributors list (or if I'd rather opt out, I've said so in the PR)

[edgarrmondragon]

The CodeFactor issues seem to be unrelated?

[edgarrmondragon]

All tests (except for Python 2.7) are 🟢 over on my fork: https://github.com/edgarrmondragon/simpleeval/actions/runs/9782703954

[edgarrmondragon]

Ping @danthedeckie