[1.11.x] Sorted imports per isort 4.2.9.

Backport of cde31daf88 from master

tests/model_options/models/tablespaces.py

@@ -1,6 +1,5 @@
 from django.db import models
 
-
 # Since the test database doesn't have tablespaces, it's impossible for Django
 # to create the tables for models where db_tablespace is set. To avoid this
 # problem, we mark the models as unmanaged, and temporarily revert them to