Add Python 3.13 to test matrix

[lilyminium]

Fixes #

Adds Python 3.13 to test matrix.

Also testing DGLs:

• dgl 2.1: works up to and including Py3.12
• dgl 2.2: works up to and including Py3.12
• dgl 2.3: works up to and including Py3.12
• dgl 2.4: not on conda-forge yet?

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.34%. Comparing base (14f6325) to head (6d617b2).
⚠️ Report is 1 commits behind head on main.

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[mattwthompson]

Still seeing some conflicts between DGL 2.3 and Python 3.13 ... hmm

[lilyminium]

Yeah, this PR was probably a little premature -- DGL isn't actually supported on Python 3.13 yet. I'm cycling through major versions 2.1-2.4 though since IIRC that was a to-do item to check -- 2.1 is now getting pretty old.