typing: Tighten type annotation for settings.CACHES.
typing: Tighten type annotation for settings.CACHES.

In zliencer.management.commands.populate_db, we assign the value of
settings.CACHES["default"] to `default_cache`.

django-stubs infers `settings.CACHES` to be `Dict[str, object]`. We make
the type specific enough so that we can access `default_cache` as a
dict.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip