backends: Fix a type: ignore issue.
backends: Fix a type: ignore issue.

I checked that this does not interfere with the MRO of the auth
backends:

In [1]: import zproject.backends; zproject.backends.GitHubAuthBackend.__mro__
Out[1]:
(zproject.backends.GitHubAuthBackend,
 zproject.backends.SocialAuthMixin,
 zproject.backends.ZulipAuthMixin,
 zproject.backends.ExternalAuthMethod,
 abc.ABC,
 social_core.backends.github.GithubOAuth2,
 social_core.backends.oauth.BaseOAuth2,
 social_core.backends.oauth.OAuthAuth,
 social_core.backends.base.BaseAuth,
 object)

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,synicalsyntax/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,brainwane/zulip,showell/zulip,kou/zulip,punchagan/zulip,kou/zulip,andersk/zulip,showell/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,kou/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,showell/zulip,brainwane/zulip,synicalsyntax/zulip,brainwane/zulip,showell/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,synicalsyntax/zulip,zulip/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,showell/zulip,synicalsyntax/zulip,rht/zulip,rht/zulip,brainwane/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,showell/zulip,kou/zulip,brainwane/zulip,zulip/zulip