[DNM] Support Python 3.13

[mattwthompson]

PR Summary:

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.36%. Comparing base (a3812dc) to head (59bfc39).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #603      +/-   ##
==========================================
- Coverage   62.04%   60.36%   -1.69%     
==========================================
  Files          16       16              
  Lines        1665     1665              
==========================================
- Hits         1033     1005      -28     
- Misses        632      660      +28     

[mattwthompson]

Looks like I'm barking down a well-barked-down tree conda-forge/openbabel-feedstock#50