gh-145921: Add "_DuringGC" functions for tp_traverse (GH-145925)

There are newly documented restrictions on tp_traverse:

    The traversal function must not have any side effects.
    It must not modify the reference counts of any Python
    objects nor create or destroy any Python objects.

* Add several functions that are guaranteed side-effect-free,
  with a _DuringGC suffix.
* Use these in ctypes
* Consolidate tp_traverse docs in gcsupport.rst, moving unique
  content from typeobj.rst there

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>