Fixed #24001 -- Added range fields for PostgreSQL.

Added support for PostgreSQL range types to contrib.postgres.

- 5 new model fields
- 4 new form fields
- New validators
- Uses psycopg2's range type implementation in python

tests/postgres_tests/test_array.py

@@ -237,6 +237,7 @@ class TestMigrations(TestCase):
         name, path, args, kwargs = field.deconstruct()
         self.assertEqual(path, 'postgres_tests.models.ArrayFieldSubclass')
 
+    @unittest.skipUnless(connection.vendor == 'postgresql', 'PostgreSQL required')
     @override_settings(MIGRATION_MODULES={
         "postgres_tests": "postgres_tests.array_default_migrations",
     })