Nested classes with SetField / SequenceField attributes

[syonekura]

Hi, first of all, thanks for developing this great library.

I have a couple of classes with the following structure:

import related


@related.immutable()
class A:
    a = related.SetField(str)


@related.immutable()
class B:
    b = related.SetField(A)


b = B(b=[
    A(a=list('asda'))
])
related.to_json(b)

When I run this code I get a TypeError:

Traceback (most recent call last):
  File "/.../fixtures/issue.py", line 15, in <module>
    A(a=list('asda'))
  File "<attrs generated init f72a7b4d1a1c90a964e7a0a277e0d59a773ff62a>", line 3, in __init__
  File "/.../virtualenvs/5Foejugn/lib/python3.6/site-packages/related/converters.py", line 90, in __call__
    args = {to_model(self.cls, value) for value in values}
  File "/.../virtualenvs/5Foejugn/lib/python3.6/site-packages/related/converters.py", line 90, in <setcomp>
    args = {to_model(self.cls, value) for value in values}
  File "<attrs generated hash 5c9557479baa8d43a71218b554b729b7a6f45a5c>", line 4, in __hash__
TypeError: unhashable type: 'TypedSet'

For some reason I cannot transform to JSON/YML a class with a Set of Sets, am I doing something wrong or is this a bug?