[1.8.x] Fixed #24893 -- Fixed lack of unique constraint when changing a field from primary_key=True to unique=True

Backport of e1e6399c2c from master

tests/schema/models.py

@@ -82,6 +82,7 @@ class BookWithSlug(models.Model):
 
 class IntegerPK(models.Model):
     i = models.IntegerField(primary_key=True)
+    j = models.IntegerField(unique=True)
 
     class Meta:
         apps = new_apps