admin readonly_fields with unique_together raises IntegrityError

[Harm Verhagen]

I'm getting an IntegrityError when editing a model in the admin, when one of the fields of a unique_together pair is in readonly_fields

example

#models.py
class Foo(models.Model):
    a = models.IntegerField()
    b = models.IntegerField()

    class Meta:
        unique_together = (('a', 'b'), )

#admin.py
class FooAdmin(admin.ModelAdmin):
    list_display = ('id', 'a', 'b')
    readonly_fields = ('b', )

steps to reproduce

In django shell create two instances

>>> from data.models import Foo
>>> Foo.objects.create(a=1,b=1)
<Foo: Foo object (1)>
>>> Foo.objects.create(a=2,b=1)
<Foo: Foo object (2)>

Then in the admin edit Foo object 1 and change a to 2. (introducing a duplicate)

expected result

A form validation error in the admin: "Foo with this A and B already exists."

actual result

A server 500 error (IntegrityError)

IntegrityError at /admin/data/foo/3/change/
duplicate key value violates unique constraint "data_foo_a_b_ac92595b_uniq"
DETAIL:  Key (a, b)=(1, 1) already exists.

notes

When removing the readonly_fields from the admin, all works as expected.

Maybe related to #13091 (similar issue when list_editable is used instead of readonly_fields)