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/debug.py

@@ -8,10 +8,10 @@ import threading
 import traceback
 import tracemalloc
 from types import FrameType
+from typing import Optional
 
 from django.conf import settings
 from django.utils.timezone import now as timezone_now
-from typing import Optional
 
 logger = logging.getLogger('zulip.debug')