Set up migration path from `boost::python`.

As boost::python is not well maintained, and lacks support for integration with basic standard library types we should try to prepare the way for easy migration away from boost::python. This means ensuring that python-exposure is done in a homogeneous and framework agnostic manner. If this can be done, then the next step is to compare with pybind11, nanobind, etc. The clean-up / refactoring should not be disruptive, and improve the code-base in any case.