Only defuse stdlib through an env var

https://github.com/inducer/relate/issues/905

slixmpp/__init__.py

@@ -4,9 +4,13 @@
 # This file is part of Slixmpp.
 # See the file LICENSE for copying permission.
 import logging
+from os import getenv
 logging.getLogger(__name__).addHandler(logging.NullHandler())
 
-# Use defusedxml if available
+# Use defusedxml if wanted
+# Since enabling it can have adverse consequences for the programs using
+# slixmpp, do not enable it by default.
+if  getenv('SLIXMPP_ENABLE_DEFUSEDXML', default='false').lower() == 'true':
     try:
         import defusedxml
         defusedxml.defuse_stdlib()