[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_operations.py

@@ -11,10 +11,10 @@ from django.test.utils import CaptureQueriesContext
 from . import PostgreSQLTestCase
 
 try:
+    from django.contrib.postgres.indexes import BrinIndex, BTreeIndex
     from django.contrib.postgres.operations import (
         AddIndexConcurrently, CreateExtension, RemoveIndexConcurrently,
     )
-    from django.contrib.postgres.indexes import BrinIndex, BTreeIndex
 except ImportError:
     pass