pylint: fix consider-iterating-dictionary

Related: https://pagure.io/freeipa/issue/9278

Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
Reviewed-By: Stanislav Levin <slev@altlinux.org>

ipaserver/custodia/message/formats.py

@@ -52,7 +52,7 @@ class Validator:
         else:
             msg_value = msg['value']
 
-        if msg_type not in list(self.types.keys()):
+        if msg_type not in self.types:
             raise UnknownMessageType("Type '%s' is unknown" % msg_type)
 
         if msg_type not in self.allowed: