Migrations keep altering unique_together

[aigarius@…]

Each new migration adds AlterUniqueTogether operation even if that has not changed.

Reproducing:

1. Create the following models.py

   class A(Model):
       bar = IntegerField()
       foo = IntegerField()
       baz = IntegerField()
       class Meta:
           unique_together = (('bar', 'foo'), )
   
   class B(Model):
       x = IntegerField()
   

2. Create an initial migration. All is good.
3. Add a field "y = IntegerField()" to B
4. Create a new auto migration.

Expected:
New migration contains only one operation: AddFiled

Actual:
New migration contains the expected AddField operation *and* the following:

        migrations.AlterUniqueTogether(
            name='a',
            unique_together=set([(b'bar', b'foo')]),

[Tim Graham]

It's a regression in #23014 which I reopened.