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>

tools/diagnose

@@ -1,17 +1,16 @@
 #!/usr/bin/env python3
-
 import os
 import platform
 import shlex
-import sys
 import subprocess
-
+import sys
 from typing import Callable, List
 
 TOOLS_DIR = os.path.dirname(__file__)
 ROOT_DIR = os.path.dirname(TOOLS_DIR)
 sys.path.insert(0, ROOT_DIR)
 from scripts.lib.zulip_tools import get_dev_uuid_var_path
+
 UUID_VAR_PATH = get_dev_uuid_var_path()
 
 def run(check_func: Callable[[], bool]) -> None:
@@ -88,7 +87,7 @@ def test_models() -> bool:
     os.environ['DJANGO_SETTINGS_MODULE'] = settings_module
     import django
     django.setup()
-    from zerver.models import UserProfile, Realm
+    from zerver.models import Realm, UserProfile
     print('Num realms: ', Realm.objects.count())
     print('Num users: ', UserProfile.objects.count())
     return True