[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_ranges.py

@@ -18,6 +18,7 @@ from .models import (
 
 try:
     from psycopg2.extras import DateRange, DateTimeTZRange, NumericRange
+
     from django.contrib.postgres import fields as pg_fields, forms as pg_forms
     from django.contrib.postgres.validators import (
         RangeMaxValueValidator, RangeMinValueValidator,