Implement hashing functions for Command and ServerSession.

evennia/server/serversession.py

@@ -433,6 +433,15 @@ class ServerSession(Session):
         except AttributeError:
             return False
 
+    def __hash__(self):
+        """
+        Python 3 requires that any class which implements __eq__ must also
+        implement __hash__ and that the corresponding hashes for equivalent
+        instances are themselves equivalent.
+
+        """
+        return hash(self.address)
+
     def __ne__(self, other):
         try:
             return self.address != other.address