[3.11] gh-93741: Add private C API _PyImport_GetModuleAttrString() (GH-93742) (GH-93792)

It combines PyImport_ImportModule() and PyObject_GetAttrString()
and saves 4-6 lines of code on every use.

Add also _PyImport_GetModuleAttr() which takes Python strings as arguments.
(cherry picked from commit 6fd4c8ec7740523bb81191c013118d9d6959bc9d)