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/data_import/import_util.py

@@ -1,20 +1,29 @@
-import random
-import requests
-import shutil
 import logging
 import os
+import random
+import shutil
 import traceback
-import ujson
+from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Tuple, TypeVar
 
-from typing import List, Dict, Any, Optional, Set, Callable, Iterable, Tuple, TypeVar
+import requests
+import ujson
 from django.forms.models import model_to_dict
 
-from zerver.models import Realm, RealmEmoji, Subscription, Recipient, \
-    Attachment, Stream, Message, UserProfile, Huddle
 from zerver.data_import.sequencer import NEXT_ID
 from zerver.lib.actions import STREAM_ASSIGNMENT_COLORS as stream_colors
 from zerver.lib.avatar_hash import user_avatar_path_from_ids
 from zerver.lib.parallel import run_parallel
+from zerver.models import (
+    Attachment,
+    Huddle,
+    Message,
+    Realm,
+    RealmEmoji,
+    Recipient,
+    Stream,
+    Subscription,
+    UserProfile,
+)
 
 # stubs
 ZerverFieldsT = Dict[str, Any]