At least in a debug build, the MSVC 64-bit compiler seems to allocate space for each unique variable declared in the function body.  Therefore, by changing the temporary variables to be named identically, the amount of required space is minimized.

The refactoring of Py_DECREF is needed to prevent an error for local variable defined before use when Py_DECREF is nested within another macro already defining _py_tmp (it would expand to roughly PyObject *_py_tmp = _py_tmp).