gh-105922: PyImport_AddModule() uses Py_DECREF()

Rewrite PyImport_AddModule() to simply call Py_DECREF(), rather than
creating a weak reference,  to get a borrowed reference to the
module.

In the documentation, add a link to sys.modules to explicit which
"modules" are checked.

Author: vstinner

Doc/c-api/import.rst

@@ -103,8 +103,8 @@ Importing Modules
    Return the module object corresponding to a module name.
 
    The *name* argument may be of the form ``package.module``. First check the
-   modules dictionary if there's one there, and if not, create a new one and
-   insert it in the modules dictionary.
+   :data:`sys.modules` dictionary if there's one there, and if not, create a
+   new one and insert it in the :data:`sys.modules` dictionary.
 
    Return a :term:`strong reference` to the module on success. Return ``NULL``
    with an exception set on failure.

Python/import.c

@@ -372,16 +372,7 @@ PyImport_AddModuleObject(PyObject *name)
     if (!mod) {
         return NULL;
     }
-
-    // gh-86160: PyImport_AddModuleObject() returns a borrowed reference
-    PyObject *ref = PyWeakref_NewRef(mod, NULL);
-    Py_DECREF(mod);
-    if (ref == NULL) {
-        return NULL;
-    }
-
-    mod = PyWeakref_GetObject(ref);
-    Py_DECREF(ref);
+    Py_DECREF(mod);  // sys.modules holds a strong reference
     return mod; /* borrowed reference */
 }