Cleanup internals for garbage-collected `py::class_` objects

[YannickJadoul]

In case we'd forget, @EricCousineau-TRI made some good remarks on #2564, that we should still attend to:

Not sure if it'd be easily addressable, but there may be lingering references from other types here:

pybind11/include/pybind11/detail/internals.h

Lines 135 to 137 in 7c71dd3

	std::vector<PyObject *(*)(PyObject *, PyTypeObject *)> implicit_conversions;
	std::vector<std::pair<const std::type_info *, void *(*)(void *)>> implicit_casts;
	std::vector<bool (*)(PyObject *, void *&)> *direct_conversions;

e.g. you're freeing type A, but type B has a conversion related to that type

---

• implicit_casts looks like it's only used for MI, so by construction I don't think you'd have a dangling parent base.
• direct_conversions looks like it's only used for NumPy dtypes
• implicit_conversions looks like it requires that both types be alive, so also should be fine. However, you may have unbounded memory growth (e.g. multiple registrations for a re-registered C++ class)

So yeah, looks like everything is fine. Just may want to make a minor note on implicit_conversiosn being unbounded; however, it's unlikely that unbounded growth will matter much? (unless someone likes running the GC every 30sec for 24hrs?)

Originally posted by @EricCousineau-TRI in #2564 (comment)