Use static type checker

[OriolAbril]

We added type hints to the library using docstub (which parses the docstrings to generate the type hints automatically if using correct numpydoc syntax). We also run docstub as part of tox -e check so docstring and type hints are always up do date.

We don't do any static type checking out of these type hints though. I think it would be great to use mypy or equivalent to check potential issues.