[3.6] bpo-31499, xml.etree: Fix xmlparser_gc_clear() crash (GH-3641) (#3645)

* bpo-31499, xml.etree: Fix xmlparser_gc_clear() crash

xml.etree: xmlparser_gc_clear() now sets self.parser to NULL to prevent a
crash in xmlparser_dealloc() if xmlparser_gc_clear() was called previously
by the garbage collector, because the parser was part of a reference cycle.

Co-Authored-By: Serhiy Storchaka <storchaka@gmail.com>
(cherry picked from commit e727d41ffcd91b21ce82026ec8c8381d34a16209)
3 files changed