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.
diff --git a/python/tests/tstmem.py b/python/tests/tstmem.py
index 6b34cf3..4e33991 100755
--- a/python/tests/tstmem.py
+++ b/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):