python: Sort migrations/management command imports with isort.

This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.

These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.

zerver/management/commands/send_webhook_fixture_message.py

@@ -1,15 +1,16 @@
 import os
-import ujson
-from typing import Union, Dict, Optional
+from typing import Dict, Optional, Union
 
+import ujson
 from django.conf import settings
 from django.core.management.base import CommandParser
 from django.test import Client
 
-from zerver.lib.management import ZulipBaseCommand, CommandError
+from zerver.lib.management import CommandError, ZulipBaseCommand
 from zerver.lib.webhooks.common import standardize_headers
 from zerver.models import get_realm
 
+
 class Command(ZulipBaseCommand):
     help = """
 Create webhook message based on given fixture