move pyexpat.ExpatError to xml.parsers.expat.ExpatError (#11168)

This matches the name reported by the cass at runtime.

related to https://github.com/python/typeshed/issues/11141

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stdlib/xml/parsers/expat/__init__.pyi

@@ -1 +1,7 @@
 from pyexpat import *
+
+# This is actually implemented in the C module pyexpat, but considers itself to live here.
+class ExpatError(Exception):
+    code: int
+    lineno: int
+    offset: int