fix return, is not

Author: jschlyter

src/cryptojwt/key_jar.py

@@ -256,7 +256,7 @@ def get_issuer_keys(self, issuer_id):
     @deprecated_alias(issuer="issuer_id", owner="issuer_id")
     def __contains__(self, issuer_id):
         _iss = self._get_issuer(issuer_id)
-        return not _iss is None
+        return _iss is not None
 
     @deprecated_alias(issuer="issuer_id", owner="issuer_id")
     def __getitem__(self, issuer_id=""):