flask + authlib, no database, configuration is extremely simple

$ git clone https://github.com/psylity/oauth2mock
$ cd oauth2mock
$ python3 -m venv venv
$ source venv/bin/activate
$ python3 -m pip install -r requirements.txt

You may edit or not app.py to adjust oauth2 server to your app. Most often it is the scope parameter.

$ flask run

Now you can navigate to https://127.0.0.1:5000. It is default url, you can modify it by specifying port/host parameters, like that:

$ flask run --host 0.0.0.0 --port 8080