check_allowed workaround not needed when jupyterhub>4.0.1 is required

[consideRatio]

I'm adding a workaround in the code, and want to reference an issue with more information. This is that issue.

The workaround looks like below and will be put in all check_allowed overrides.

    async def check_allowed(self, username, auth_model):
        # workaround for JupyterHub<=4.0.1, see <this issue>
        if auth_model is None:
            return True

In JupyterHub<=4.0.1, check_allowed was sometimes called directly by JupyterHub passing auth_model as None. Me and Min concluded that it was fine to just return True in those cases for the OAuthenticator classes in https://github.com/jupyterhub/jupyterhub/issues/4461.