[3.14] gh-132775: Fix Interpreter.call() __main__ Visibility (gh-135638)

As noted in the new tests, there are a few situations we must carefully accommodate
for functions that get pickled during interp.call().  We do so by running the script
from the main interpreter's __main__ module in a hidden module in the other
interpreter.  That hidden module is used as the function __globals__.

(cherry picked from commit 269e19e0a7, AKA gh-135595)

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

Python/pystate.c

@@ -1251,6 +1251,7 @@ _Py_CheckMainModule(PyObject *module)
         PyObject *msg = PyUnicode_FromString("invalid __main__ module");
         if (msg != NULL) {
             (void)PyErr_SetImportError(msg, &_Py_ID(__main__), NULL);
+            Py_DECREF(msg);
         }
         return -1;
     }