Compatibility with MyPy

### Things to check first

- [x] 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