Fix regression in unique_together validation with SerializerMethodField

[kalekseev]

While building validator unique together fields were compared with declared field names instead of model field names

fixes #9700

closes #9710

[auvipy]

looks good to me

[kalekseev]

It also possible to unify code related to get_unique_together_constraints, these two code blocks are basically the same

django-rest-framework/rest_framework/serializers.py

Lines 1600 to 1604 in 33d59fe

	for unique_together, queryset, condition_fields, condition in self.get_unique_together_constraints(self.Meta.model):
	# Skip if serializer does not map to all unique together sources
	unique_together_and_condition_fields = set(unique_together) | set(condition_fields)
	if not set(source_map).issuperset(unique_together_and_condition_fields):
	continue

django-rest-framework/rest_framework/serializers.py

Lines 1475 to 1478 in 33d59fe

	for unique_together_list, queryset, condition_fields, condition in self.get_unique_together_constraints(model):
	unique_together_list_and_condition_fields = set(unique_together_list) | set(condition_fields)
	if set(field_names).issuperset(unique_together_list_and_condition_fields):
	unique_constraint_names |= unique_together_list_and_condition_fields

[auvipy]

you can come with the unified code for review as well

[browniebroke]

From my perspective, I'm happy to get it merge this as is. Thanks for picking it up