is_encrypted doesn't require the plugin to be instanciated

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

slixmpp_omemo/__init__.py

@@ -659,7 +659,8 @@ class XEP_0384(BasePlugin):
 
         return await self._omemo().getTrustForJID(jid.bare)
 
-    def is_encrypted(self, msg: Message) -> bool:
+    @classmethod
+    def is_encrypted(cls, msg: Message) -> bool:
         return msg.xml.find('{%s}encrypted' % OMEMO_BASE_NS) is not None
 
     async def decrypt_message(