Function from_python

tests/test_callbacks.cpp:44–48

        borrow<callable>((PyObject *) context)(arg);
    }
    bool from_python(handle src, uint8_t, cleanup_list*) noexcept {
        if (!isinstance<callable>(src)) return false;
        value = {(void *) src.ptr(), &wrap_call};
        return true;
    }
    static handle from_cpp(callback cb, rv_policy policy, cleanup_list*) noexcept {
        if (cb.func == &wrap_call)
            return handle((PyObject *) cb.context).inc_ref();