mypy v0.98 and later no longer allows implicit optional

[masa-08]

The mypy 0.98 and later does not allow implicit options default, so some typechecks fail.

The following is what fails.

• https://github.com/dbader/schedule/blob/master/schedule/__init__.py#L226
• https://github.com/dbader/schedule/blob/master/schedule/__init__.py#L468

The following is mypy reference. https://mypy.readthedocs.io/en/stable/config_file.html#none-and-optional-handling