gh-120170: Exclude __mp_main__ in C version of whichmodule()

Importing multiprocessing adds an alias to __main__ named __mp_main__.
In #23403, the Python version of whichmodule() was fixed to exclude
__mp_main__. Apply the same fix to the C version of the function.

Author: li-dan

Misc/NEWS.d/next/Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst

@@ -0,0 +1,3 @@
+Fix an issue in the :mod:`!_pickle` extension module in which importing
+:mod:`multiprocessing` could change how pickle identifies which module an
+object belongs to, potentially breaking the unpickling of those objects.

Modules/_pickle.c

@@ -1888,6 +1888,10 @@ _checkmodule(PyObject *module_name, PyObject *module,
             _PyUnicode_EqualToASCIIString(module_name, "__main__")) {
         return -1;
     }
+    if (PyUnicode_Check(module_name) &&
+            _PyUnicode_EqualToASCIIString(module_name, "__mp_main__")) {
+        return -1;
+    }
 
     PyObject *candidate = get_deep_attribute(module, dotted_path, NULL);
     if (candidate == NULL) {