Because making & testing my own string caster is not something I'd want
to do right now (seems complicated by looking at what's all done for
std::string), the functions currently all go through a std::string
instead.
Pybind changed py::module to py::module_ in order to support C++
modules, adapting to that change. It still builds on older versions,
but we're using only the new APIs.
Vladimír Vondruš