Add support for py::issubclass

[markuspi]

pybind11 is missing a py::issubclass functionality, analogous to py::isinstance.

I am thinking of adding something like this to pytypes.h:

template <typename T>
bool issubclass(handle cls) {
    const auto parent = type::handle_of<T>();
    return issubclass(cls, parent);
}

inline bool issubclass(handle cls, handle parent) {
    const auto result = PyObject_IsSubclass(cls.ptr(), parent.ptr());
    if (result == -1)
        throw error_already_set();
    return result != 0;
}

However, I am not sure this will work for subclasses of object, as they are also handled seperately in py::isinstance.
I would appreciate your feedback!

[damienmarchal]

Is there any updates on this conversation ?
I may have some use for such feature.