Fixed #29824 -- Added support for database exclusion constraints on PostgreSQL.

Thanks to Nick Pope and Mariusz Felisiak for review.

Co-Authored-By: Mariusz Felisiak <felisiak.mariusz@gmail.com>

tests/postgres_tests/models.py

@@ -183,3 +183,15 @@ class NowTestModel(models.Model):
 
 class UUIDTestModel(models.Model):
     uuid = models.UUIDField(default=None, null=True)
+
+
+class Room(models.Model):
+    number = models.IntegerField(unique=True)
+
+
+class HotelReservation(PostgreSQLModel):
+    room = models.ForeignKey('Room', on_delete=models.CASCADE)
+    datespan = DateRangeField()
+    start = models.DateTimeField()
+    end = models.DateTimeField()
+    cancelled = models.BooleanField(default=False)