Indicate that this is a typed package

[PeterJCLaw]

This allows type checkers to make use of the types from this package when validating consuming code.

Fixes https://github.com/attwad/python-osc/issues/155

[PeterJCLaw]

Note that it might be worth adding type checking (perhaps with mypy) before merging this, to ensure that the types being published are correct and self-consistent.

[PeterJCLaw]

Note that it might be worth adding type checking (perhaps with mypy) before merging this, to ensure that the types being published are correct and self-consistent.

I've added such checking & fixed the discovered issues in #157.

[attwad]

thank you!