Examples don't work in Python 3.6

[necaris]

See #10 -- while the examples work fine in Python 3.7, it would be ideal if we could make them work (or document where they don't) in 3.6

[dantheman39]

The issue seems to be here: https://github.com/upsidetravel/graphene-pydantic/blob/f0326a75c7c3e7b0b5dfa1aa58d0fd864da44705/examples/departments.py#L35

class DepartmentModel(pydantic.BaseModel):
    id: uuid.UUID
    name: str
    employees: T.List[T.Union[ManagerModel, EmployeeModel]]

employees is a Union of Manager and Employee, and Manager is a subclass of Employee. In 3.6, typing.py throws away direct subclasses in Unions, so in DepartmentModel.__annotations__, the employees field is listed as typing.List[__main__.EmployeeModel], and not typing.List[typing.Union[__main__.ManagerModel, __main__.EmployeeModel]] as in 3.7, so all of the introspection that we rely on has no way of knowing that employees is a Union of Manager and Employee, it just thinks it's an employee.

In the documentation for Union they explicitly note that subclasses are no longer thrown out as of python 3.7.

[necaris]

Closed by #12