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/error_notify.py

@@ -1,16 +1,17 @@
 # System documented in https://zulip.readthedocs.io/en/latest/subsystems/logging.html
-
 from collections import defaultdict
+from typing import Any, Dict, Optional
+
 from django.conf import settings
 from django.core.mail import mail_admins
 from django.http import HttpResponse
 from django.utils.translation import ugettext as _
-from typing import Any, Dict, Optional
 
 from zerver.filters import clean_data_from_query_parameters
-from zerver.models import get_stream, get_system_bot
 from zerver.lib.actions import internal_send_stream_message
-from zerver.lib.response import json_success, json_error
+from zerver.lib.response import json_error, json_success
+from zerver.models import get_stream, get_system_bot
+
 
 def format_email_subject(email_subject: str) -> str:
     """