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/server_initialization.py

@@ -1,10 +1,10 @@
+from typing import Iterable, Optional, Tuple
+
 from django.conf import settings
 
 from zerver.lib.bulk_create import bulk_create_users
-from zerver.models import Realm, UserProfile, email_to_username, get_client, \
-    get_system_bot
+from zerver.models import Realm, UserProfile, email_to_username, get_client, get_system_bot
 
-from typing import Iterable, Optional, Tuple
 
 def server_initialized() -> bool:
     return Realm.objects.exists()