gh-140123: Make ElementTree.find use registered global namespaces

ElementTree.find, findall, findtext, and iterfind now use the
global _namespace_map as fallback when no namespaces argument
is provided. Fixes inconsistencies between ElementTree and
ElementPath when using ET.register_namespace.

Author: sangampaudel530

Lib/test/test_xml_etree.py

@@ -21,6 +21,7 @@
 import unittest.mock as mock
 import warnings
 import weakref
+import xml.etree.ElementTree as ET
 
 from contextlib import nullcontext
 from functools import partial