[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)