[3.1.x] Bumped minimum isort version to 5.1.0.

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

Backport of e74b3d724e from master

tests/postgres_tests/test_json_deprecation.py

@@ -1,7 +1,9 @@
 try:
-    from django.contrib.postgres.fields import JSONField
-    from django.contrib.postgres.fields.jsonb import KeyTransform, KeyTextTransform
     from django.contrib.postgres import forms
+    from django.contrib.postgres.fields import JSONField
+    from django.contrib.postgres.fields.jsonb import (
+        KeyTextTransform, KeyTransform,
+    )
 except ImportError:
     pass