gh-139400: Make sure that parent parsers outlive their subparsers in `pyexpat` (#139403)

* Modules/pyexpat.c: Disallow collection of in-use parent parsers.

Within libexpat, a parser created via `XML_ExternalEntityParserCreate`
is relying on its parent parser throughout its entire lifetime.
Prior to this fix, is was possible for the parent parser to be
garbage-collected too early.