[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_apps.py

@@ -8,6 +8,7 @@ try:
     from psycopg2.extras import (
         DateRange, DateTimeRange, DateTimeTZRange, NumericRange,
     )
+
     from django.contrib.postgres.fields import (
         DateRangeField, DateTimeRangeField, IntegerRangeField,
     )