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/convert_slack_data.py

@@ -3,10 +3,12 @@ import os
 import tempfile
 from typing import Any
 
-from django.core.management.base import BaseCommand, CommandParser, CommandError
+from django.core.management.base import BaseCommand, CommandError, \
+    CommandParser
 
 from zerver.data_import.slack import do_convert_data
 
+
 class Command(BaseCommand):
     help = """Convert the Slack data into Zulip data format."""