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/presence.py

@@ -1,21 +1,14 @@
-from collections import defaultdict
-
 import datetime
 import itertools
 import time
+from collections import defaultdict
+from typing import Any, Dict, List, Set
 
 from django.utils.timezone import now as timezone_now
 
-from typing import Any, Dict, List, Set
-
 from zerver.lib.timestamp import datetime_to_timestamp
-from zerver.models import (
-    query_for_ids,
-    PushDeviceToken,
-    Realm,
-    UserPresence,
-    UserProfile,
-)
+from zerver.models import PushDeviceToken, Realm, UserPresence, UserProfile, query_for_ids
+
 
 def get_status_dicts_for_rows(all_rows: List[Dict[str, Any]],
                               mobile_user_ids: Set[int],