add support for unfolding to python schema

Is your feature request related to a problem? Please describe.

adding _unfoldable = [] does nothing when creating a schema with the python client.
E.g.:

myschema = Schema()


class Method(DocumentTemplate):
    _schema = myschema
    linked = OtherClass
    _unfoldable = [] #

Expected output: The resulting schema has @unfoldable in the document definition, but it hasn't.

Describe the solution you'd like

for unfoldable, it seems that it could just function analogous to _schema or _abstract. so _unfoldable = [].

Describe alternatives you've considered
Decorator.

Additional context
Im trying to use terminus exclusively from python (i.e. i'd like to be able to access all functionality in a pythonic way, including retrieval -> need to be able to have unfoldable for the db to return a subgraph/tree instead of only a list of solutions).

Start by tracing how Python schema construction handles the existing _schema and _abstract attributes, then compare that path with document-definition generation. Implement support for _unfoldable = [] so the resulting definition includes @unfoldable, and verify that the generated schema supports retrieval of the requested subgraph or tree.