Add "py.typed" to support type checking

[roberfi]

When mypy is executed with alive_progress import and ignore_missing_imports to false, the following error is diplayed:

error: Skipping analyzing "alive_progress": module is installed, but missing library stubs or py.typed marker  [import-untyped]
note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

According to PEP-561, package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing.

So, it would be very useful if "py.typed" file is added to the project and package.