Add public API for type checkers

[alkatar21]

The code works like this, but at least pylance can recognize that there is Timer, but does not recognize it as part of the public API:

Adding the following to the __init__.py would solve this.

# Use __all__ to let type checkers know what is part of the public API.
__all__ = ('Timer', 'TimerError')

I could make a PR for this. But then the question would arise whether __all__ should also be added in the modules and whether relative imports should be used in general?

[gahjelle]

Thanks for the report. If you have the capacity, I'd be grateful for a PR.

• I think the usual practice is to use a list and not a tuple when specifying __all__ (https://docs.python.org/3/tutorial/modules.html?highlight=__all__#importing-from-a-package)
• I don't think you need to define __all__ in _timer.py and _timers.py. They're not part of the public API.
• I'm not sure I understand what you mean by "whether relative imports should be used in general?" I definitely prefer absolute imports over relative imports.

Thanks again!