Properly visit self in >=3.9 traverse

[Skylion007]

Description

• This PR fixes a small bug that was already fixed in nanobind. Specifically, the our traverse behavior does not follow the clarified behavior of the tp-traverse function Python >=3.9.
• I originally discovered this bug when researching the 3.11 fixes, but this did not fix the root cause there. Still, it's better to have proper behavior for >= 3.9 so this PR cherry picks that fix and ensure we properly traverse the classes when calling gc on dynamically allocated classes.

Suggested changelog entry:

* Ensure proper behavior when garbage collecting classes with dynamic attributes in Python >=3.9.

[rwgk]

Optional: I'd add this comment (your link from the PR description) so the next person to look here has an easier time understanding this code:

// https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_traverse

[rwgk]

Thanks @Skylion007, I'll go ahead merging this PR, to get it into the next smart_holder update.