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>

zproject/dev_urls.py

@@ -1,16 +1,18 @@
+import os
 from urllib.parse import urlsplit
-from django.conf.urls import url
+
 from django.conf import settings
+from django.conf.urls import url
 from django.conf.urls.static import static
 from django.contrib.staticfiles.views import serve as staticfiles_serve
 from django.http import HttpRequest, HttpResponse
 from django.views.generic import TemplateView
-import os
 from django.views.static import serve
-import zerver.views.development.registration
+
 import zerver.views.auth
 import zerver.views.development.email_log
 import zerver.views.development.integrations
+import zerver.views.development.registration
 
 # These URLs are available only in the development environment