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/test_helpers.py

@@ -1,57 +1,68 @@
 from contextlib import contextmanager
 from typing import (
-    Any, Callable, Dict, Generator, Iterable, Iterator, List, Mapping,
-    Optional, Tuple, Union, IO, TypeVar, TYPE_CHECKING,
+    IO,
+    TYPE_CHECKING,
+    Any,
+    Callable,
+    Dict,
+    Generator,
+    Iterable,
+    Iterator,
+    List,
+    Mapping,
+    Optional,
+    Tuple,
+    TypeVar,
+    Union,
 )
 
-from django.urls import URLResolver
-from django.conf import settings
-from django.test import override_settings
-from django.http import HttpResponse, HttpResponseRedirect
-from django.db.migrations.state import StateApps
 import boto3
 from boto3.resources.base import ServiceResource
+from django.conf import settings
+from django.db.migrations.state import StateApps
+from django.http import HttpResponse, HttpResponseRedirect
+from django.test import override_settings
+from django.urls import URLResolver
 
 import zerver.lib.upload
+from zerver.lib import cache
 from zerver.lib.actions import do_set_realm_property
-from zerver.lib.upload import S3UploadBackend, LocalUploadBackend
 from zerver.lib.avatar import avatar_url
 from zerver.lib.cache import get_cache_backend
 from zerver.lib.db import Params, ParamsT, Query, TimeTrackingCursor
-from zerver.lib import cache
-from zerver.tornado import event_queue
-from zerver.tornado.handlers import allocate_handler_id
-from zerver.worker import queue_processors
 from zerver.lib.integrations import WEBHOOK_INTEGRATIONS
-
+from zerver.lib.upload import LocalUploadBackend, S3UploadBackend
 from zerver.models import (
-    get_realm,
-    get_stream,
     Client,
     Message,
     Realm,
     Subscription,
     UserMessage,
     UserProfile,
+    get_realm,
+    get_stream,
 )
-
-from zproject.backends import ExternalAuthResult, ExternalAuthDataDict
+from zerver.tornado import event_queue
+from zerver.tornado.handlers import allocate_handler_id
+from zerver.worker import queue_processors
+from zproject.backends import ExternalAuthDataDict, ExternalAuthResult
 
 if TYPE_CHECKING:
     # Avoid an import cycle; we only need these for type annotations.
     from zerver.lib.test_classes import ZulipTestCase, MigrationsTestCase
 
 import collections
-from unittest import mock
 import os
 import re
 import sys
 import time
-import ujson
-from moto import mock_s3
+from unittest import mock
 
 import fakeldap
 import ldap
+import ujson
+from moto import mock_s3
+
 
 class MockLDAP(fakeldap.MockLDAP):
     class LDAPError(ldap.LDAPError):