python: Sort imports with isort.

Fixes #2665.

Regenerated by tabbott with `lint --fix` after a rebase and change in
parameters.

Note from tabbott: In a few cases, this converts technical debt in the
form of unsorted imports into different technical debt in the form of
our largest files having very long, ugly import sequences at the
start.  I expect this change will increase pressure for us to split
those files, which isn't a bad thing.

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

zerver/lib/stream_subscription.py

@@ -1,12 +1,9 @@
 from typing import Any, Dict, List, Tuple
 
 from django.db.models.query import QuerySet
-from zerver.models import (
-    Recipient,
-    Stream,
-    Subscription,
-    UserProfile,
-)
+
+from zerver.models import Recipient, Stream, Subscription, UserProfile
+
 
 def get_active_subscriptions_for_stream_id(stream_id: int) -> QuerySet:
     # TODO: Change return type to QuerySet[Subscription]