Workaround for blocked PyObject_GenericSetAttr in metatypes

Python 3.14 introduced a new assertion that prevents us from using
PyObject_GenericSetAttr directly in our meta type. To work around
this, we manipulate the type dict directly.

This workaround is a simplified variant of Cython's workaround from
https://github.com/cython/cython/pull/6325.

The relevant Python change is in
https://github.com/python/cpython/pull/118454