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

[miss-islington]

• 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.
(cherry picked from commit 6edb2dd)

Co-authored-by: Sebastian Pipping sebastian@pipping.org

• Issue: Heap-use-after-free issue in pyexpat related to .ExternalEntityParserCreate #139400