mypy: Migrate from mypy_extensions to typing_extensions.

This gives us access to typing_extensions.Deque, which was not added
to typing until 3.5.4.

(PROVISION_VERSION is not bumped because the transitive dependency set
in dev.txt hasn’t changed.)

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/lib/stream_subscription.py

@@ -1,5 +1,5 @@
 from typing import Any, Dict, List, Tuple
-from mypy_extensions import TypedDict
+from typing_extensions import TypedDict
 
 from django.db.models.query import QuerySet
 from zerver.models import (