Add MyPy

[matin]

There are four failing checks that need to be resolved before MyPy can be enabled again:

cuenca/resources/base.py:18: error: Too many arguments for "Resource"
cuenca/resources/transfers.py:72: error: Incompatible return value type (got "Resource", expected "Transfer")
cuenca/resources/api_keys.py:30: error: Incompatible return value type (got "Resource", expected "ApiKey")
cuenca/resources/api_keys.py:43: error: Incompatible return value type (got "Resource", expected "ApiKey")
Found 4 errors in 3 files (checked 12 source files)

[felipao-mx]

we can solve many of those errors if we use generics:

T = TypeVar('T')
class Resource(Generic[T]):
    @classmethod
    def from_dict(cls, obj_dict: Dict) -> T:
        ...

class Creatable(Generic[T]):
    def create(cls, **kwargs) -> T:
        resp = session.post(cls._endpoint, data)
        return cls.from_dict(resp)

class Transfer(Resource['Transfer'], Creatable['Transfer']):
    ...

[matin]

@rogelioLpz