[3.12] gh-139400: Make sure that parent parsers outlive their subparsers in `pyexpat` (GH-139403) (GH-139609)

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.

(cherry picked from commit 6edb2ddb5f3695cf4938979d645f31d7fba43ec8)