[3.12] gh-105227: Add PyType_GetDict() (GH-105747) (#106600)

gh-105227: Add PyType_GetDict() (GH-105747)

This compensates for static builtin types having `tp_dict` set to `NULL`.

(cherry picked from commit a840806d338805fe74a9de01081d30da7605a29f)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>