[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_constraints.py

@@ -9,10 +9,12 @@ from . import PostgreSQLTestCase
 from .models import HotelReservation, RangesModel, Room
 
 try:
-    from django.contrib.postgres.constraints import ExclusionConstraint
-    from django.contrib.postgres.fields import DateTimeRangeField, RangeBoundary, RangeOperators
-
     from psycopg2.extras import DateRange, NumericRange
+
+    from django.contrib.postgres.constraints import ExclusionConstraint
+    from django.contrib.postgres.fields import (
+        DateTimeRangeField, RangeBoundary, RangeOperators,
+    )
 except ImportError:
     pass