Support Python 3.13

[mattwthompson]

Is your feature request related to a problem?

Python 3.13 has been out for about 14 months and we ought to look closely at its support. Currently it can be installed as such (micromamba create --name foo "openff-nagl" "python=3.13" does not error) but it's not clear if that actually works or how much.

Describe the solution you'd like

First add it to CI then evaluate whether or not it packaging should be updated

Describe alternatives you've considered

Just don't use Python 3.13

Additional context

A refactor away from DGL would impact this (probably making it easier)

Lily already took a stab at this and DGL does seem to be the crux of the issue: #221

[lilyminium]

Yeah this is mostly on dgl: dmlc/dgl#7906

We could add a version for inference only that does not require dgl, though.

[mattwthompson]

If DGL is only needed for training, we could buy time by making it an optional dependency and not installed by default for inference. That's not ideal but also much less work