Here is a patch adding Py_SAFE_DOWNCAST(). For update_star_args(), I changed the type instead, because I prefer to avoid Py_SAFE_DOWNCAST() when possible.

Modify PyEval_EvalCodeEx() and PyCode_New() to use Py_ssize_t would be more correct, but it may be slower if Py_ssize_t is larger than int, and I hope that nobody calls functions with more than INT_MAX parameters! It would be completly inefficient!