Compatibility with MyPy

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

add py.typed file in the root folder of apscheduler distribution package. Since this was requested 3 years ago (https://github.com/agronholm/apscheduler/issues/648) and declined in favor of upgrading to the v4 version and the v4 is not out yet, could it please be added in the meantime?

Use case

would be great to have type checking