mypy support

[juanitorduz]

When trying to work on #74 I felt type hints would have been very helpful. Also, they can help with developing and testing. As this code base is relatively small I think it would be nice to add type hints and add mypy checks in the tests. I would be happy to kick this off is you agree it is useful.

[aloctavodia]

Thanks for offering help. Yes, I think this could be a good addition.

[juanitorduz]

shall we close this one now :) ?