Bumped minimum isort version to 5.1.0.

Fixed inner imports per isort 5.
isort 5.0.0 to 5.1.0 was unstable.

tests/backends/postgresql/test_creation.py

@@ -13,6 +13,7 @@ except ImportError:
     pass
 else:
     from psycopg2 import errorcodes
+
     from django.db.backends.postgresql.creation import DatabaseCreation