With owner-only consumers, you cannot use the grant process:
https://www.mediawiki.org/wiki/OAuth/Owner-only_consumers you can use the
resource directly. When you request an owner-only token, you should get an
access token at that point. Unfortunately (afaik) there is no way to test
the workflow without a getting a real consumer.
You got no response token because you got an error response instead. As
stated at the link in the error message, owner-only consumers are
pre-authorized so the client should be configured with the token key and
secret directly instead of using the authorization endpoints.
I don't know how to do that using Flask; if you need help with that you'd
probably do better to ask in a forum specific to Flask.