python: Convert "".format to Python 3.6 f-strings.

Generated by pyupgrade --py36-plus --keep-percent-format, but with the
NamedTuple changes reverted (see commit
ba7906a3c6, #15132).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/openapi/openapi.py

@@ -185,7 +185,7 @@ def validate_object(content: Dict[str, Any], schema: Dict[str, Any]) -> None:
         # If the object is not opaque then continue statements
         # will be executed above and this will be skipped
         if expected_type is dict:
-            raise SchemaError('Opaque object "{}"'.format(key))
+            raise SchemaError(f'Opaque object "{key}"')
     # Check that at least all the required keys are present
     if 'required' in schema:
         for req_key in schema['required']: