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/tests/test_submessage.py

@@ -1,17 +1,10 @@
+from typing import Any, Dict, List
 from unittest import mock
 
+from zerver.lib.message import MessageDict
 from zerver.lib.test_classes import ZulipTestCase
+from zerver.models import Message, SubMessage
 
-from zerver.lib.message import (
-    MessageDict,
-)
-
-from zerver.models import (
-    Message,
-    SubMessage,
-)
-
-from typing import Any, Dict, List
 
 class TestBasics(ZulipTestCase):
     def test_get_raw_db_rows(self) -> None: