[timgraham]

This is a sketch of changes needed to the built-in serializers to support embedded model fields.

The changes aren't intended to be merged to this Django fork, rather the goal is to add these hooks upstream in Django so that we don't need to fork the serializers.

Upstream PRs:

• Refs #36986 -- Replaced yaml's Serializer._value_from_field() time handling with a representer. django/django#21054
• Fixed #37023 -- Made XML serializer put each ManyToManyField object on its own line. django/django#21056
• Added support for nested fields to XML deserializer. django/django#21057
• Fixed #36986 -- Added Field.serialize_to_python/xml() and deserialize_from_python/xml() hooks. django/django#21063