`GufeTokenizable`s cannot contain a dictionary with other `GufeTokenizable`s as keys

[ianmkenney]

Defining and instantiating a GufeTokenizable that contains a dictionary whose keys are also GufeTokenizable instances raises a TypeError. Since GufeTokenizable instances are unique and hashable, this should be supported. Additionally, such a data-structure has value in mitigating risk of stale GufeKey instances within a GufeTokenizable on deserialization (see the transformation_results_map attribute of AlchemicalArchive in #687).

from gufe.tokenization import GufeTokenizable

class Foo(GufeTokenizable):

    def __init__(self, data = None):
        self.data = data

    @staticmethod
    def _defaults(): return {}

    def _to_dict(self): return {"data": self.data}

    @classmethod
    def _from_dict(cls, dct): return cls(**dct)


if __name__ == "__main__":
    gt_attr = Foo(data=Foo())
    dct_attr_gt_value = Foo(data={"some_key": Foo()})
    dct_attr_gt_key = Foo(data={Foo(): "some_value"})

../env/bin/python gt_as_keys.py 
Traceback (most recent call last):
  File "[...]/gufe/network/gt_as_keys.py", line 20, in <module>
    dct_attr_gt_key = Foo(data={Foo(): "some_value"})
  File "[...]/gufe/gufe/tokenization.py", line 78, in __call__
    key = instance.key
          ^^^^^^^^^^^^
  File "[...]/gufe/gufe/tokenization.py", line 424, in key
    token = self._gufe_tokenize()
  File "[...]/gufe/gufe/tokenization.py", line 356, in _gufe_tokenize
    return tokenize(self)
  File "[...]/gufe/gufe/tokenization.py", line 1240, in tokenize
    dumped = json.dumps(
        obj.to_keyed_dict(include_defaults=False),
        sort_keys=True,
        cls=JSON_HANDLER.encoder,
    )
  File "[...]/gufe/env/lib/python3.13/json/__init__.py", line 242, in dumps
    **kw).encode(obj)
          ~~~~~~^^^^^
  File "[...]/gufe/env/lib/python3.13/json/encoder.py", line 202, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "[...]/gufe/env/lib/python3.13/json/encoder.py", line 263, in iterencode
    return _iterencode(o, 0)
TypeError: keys must be str, int, float, bool or None, not Foo