tests: use 'not in' for set membership test

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>

Author: jk-ozlabs

tests/mctpenv/__init__.py

@@ -439,7 +439,7 @@ async def send_control(self, sock, cmd):
             addr.set_ext(self.iface.ifindex, self.lladdr)
 
         key = (typ, cmd.iid)
-        assert not key in self.commands
+        assert key not in self.commands
 
         self.commands[key] = cmd