global: use defusedxml if it is available

setup.py

@@ -87,6 +87,7 @@ setup(
     extras_require={
         'XEP-0363': ['aiohttp'],
         'XEP-0444 compliance': ['emoji'],
+        'Safer XML parsing': ['defusedxml'],
     },
     classifiers=CLASSIFIERS,
     cmdclass={'test': TestCommand}

slixmpp/__init__.py

@@ -6,6 +6,13 @@
 import logging
 logging.getLogger(__name__).addHandler(logging.NullHandler())
 
+# Use defusedxml if available
+try:
+    import defusedxml
+    defusedxml.defuse_stdlib()
+except ImportError:
+    pass
+
 from slixmpp.stanza import Message, Presence, Iq
 from slixmpp.jid import JID, InvalidJID
 from slixmpp.xmlstream.stanzabase import ET, ElementBase, register_stanza_plugin