Dicts without full type annotations error on parse

[illeatmyhat]

@dataclass_json
@dataclass
class Foo:
    foo: Dict

Foo.from_json(Foo.to_json())

results in an error at from_json.
This is caused by missing type annotations, where Dict should be Dict[Any, Any]--though for parsing purposes it is more likely to end up being Dict[str,str]

In any case, the error produced in this case is very unfriendly and probably an easy mistake to make.

[lidatong]

hi, i agree this is probably a confusing error, see #206 and #207 for efforts at better errors!