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>

manage.py

@@ -1,7 +1,8 @@
 #!/usr/bin/env python3
+import configparser
 import os
 import sys
-import configparser
+
 if sys.version_info <= (3, 0):
     print("Error: Zulip is a Python 3 project, and cannot be run with Python 2.")
     print("Use e.g. `/path/to/manage.py` not `python /path/to/manage.py`.")
@@ -37,6 +38,7 @@ if __name__ == "__main__":
     from django.conf import settings
     from django.core.management import execute_from_command_line
     from django.core.management.base import CommandError
+
     from scripts.lib.zulip_tools import log_management_command
 
     log_management_command(" ".join(sys.argv), settings.MANAGEMENT_LOG_PATH)