[1.8.x] Sorted imports with isort; refs #23860.

Backport of 0ed7d15563 from master

tests/proxy_models/models.py

@@ -7,6 +7,7 @@ providing a modified interface to the data from the base class.
 from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
+
 # A couple of managers for testing managing overriding in proxy model cases.