Instead of using:
ClientXMPP(jid, password, plugin_config={
'feature_mechanisms': {'use_mech': 'SOME-MECH'}})
You can use:
ClientXMPP(jid, password, sasl_mech='SOME-MECH')
If you need to change the mechanism after instantiation, use:
xmpp['feature_mechanisms'].sasl.mech = 'SCRAM-MD5'