In 3.5.2, code using luigi fails mypy type checks

[ccmaymay]

The mypy type checker accepted the following code before 3.5.2:

import luigi  # type: ignore


def process_string(s: str) -> str:
    return s


class Streams(luigi.Task):
    param = luigi.Parameter()

    def run(self) -> None:
        with self.output().open('w') as output:
            output.write(process_string(self.param))

    def output(self) -> luigi.LocalTarget:
        return luigi.LocalTarget('data.txt')

But with the addition of py.typed in 3.5.2, mypy complains:

mwe.py:13: error: Argument 1 to "process_string" has incompatible type "Parameter"; expected "str"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

At runtime in the user code, the param member isn't an instance of Parameter; rather, it has been overwritten here with the parsed, normalized parameter value, which has the expected type.

Is there any guidance on writing type-checked code in 3.5.2? Or will type compliance be added later? Otherwise, could the addition of py.typed be reverted?

Thank you!