add py.typed marker for typing

[fridex]

Changes proposed in this pull request:

• see https://peps.python.org/pep-0561/
• related Add type annotations and mypy validation #1111
• related Take advantage of type hints #1557

CC @RobbeSneyders

[RobbeSneyders]

Thanks @fridex!

There were some doubts previously if this is the best way to go if our types are not perfect, see this comment. Do you have a view on that?

[fridex]

There were some doubts previously if this is the best way to go if our types are not perfect, see this comment. Do you have a view on that?

I've tried mypy on codebase and, sadly, it does not seem to be ready for typing.

[RobbeSneyders]

Can you expand a bit?

Note that we are running mypy on every commit already.