This finalises the migration to the black code style by applying double-quotes.

Rationale: Adding logic to decide on the login view to use, and
reliably disable the default login view if 2fa is enabled, adds
unnecessary complexity, compared to django-two-factor-auth being
a quite lightweight dependency.

https://github.com/rochacbruno/dynaconf/issues/251

Revert until upstream releases fix!

Works around a bug in dynaconf: Variable names in settings module must
not be the same as top-level names in the config file, or they will
magically be overridden.