tstmem.py: Try importing from libxmlmods.libxml2mod if needed

Distutils builds place libxml2mod.pyd under the libxmlmods subdir, so try this
directory if 'import libxml2mod' failed.

python/tests/tstmem.py

@@ -1,6 +1,11 @@
 #!/usr/bin/python -u
 import libxml2
-import libxml2mod
+
+try:
+    import libxml2mod
+except ModuleNotFoundError:
+    from libxmlmods import libxml2mod
+
 import sys
 
 def error(msg, data):