Fix type hint for DjangoObjectTypeOptions.model

[belkka]

Proper type is typing.Type[Model], not Model

[belkka]

Also, according to usage,

graphene-django/graphene_django/types.py

Line 192 in 9f26772

if fields and fields != ALL_FIELDS and not isinstance(fields, (list, tuple)):

there could be added type hints for fields and exclude:

class DjangoObjectTypeOptions(ObjectTypeOptions):
    ...
    fields = None  # type: Union[List[str], Tuple[str, ...], Literal['__all__']]
    exclude = None  # type: Union[List[str], Tuple[str, ...]]

This would fix mismatching type hint inherited from ObjectTypeOptions.fields, which is annotated as following in graphene:
fields = None # type: Dict[str, Field]

typing.Literal is new in python3.8, so could use str instead of Literal['__all__']