dict_object.keys() is not required for *in* operator

cleanup of .keys() from dict_object.keys() *in* operator

Change-Id: I76b92514c6740a57bc7134015ac6197393913b25
diff --git a/patrole_tempest_plugin/policy_authority.py b/patrole_tempest_plugin/policy_authority.py
index e0a26a3..d5519cf 100644
--- a/patrole_tempest_plugin/policy_authority.py
+++ b/patrole_tempest_plugin/policy_authority.py
@@ -231,7 +231,7 @@
         whether the given role is contained in context_is_admin. If it is not
         in the policy file, then default to context_is_admin: admin.
         """
-        if 'context_is_admin' in self.rules.keys():
+        if 'context_is_admin' in self.rules:
             return self._allowed(
                 access=self._get_access_token(roles),
                 apply_rule='context_is_admin')