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Yea, I looked into `ceval.c` and the function is *called incorrectly*, so there is undefined behaviour there - it has been wrong all along, in 3.5 all the way down to 2-something
 if (flags & (METH_NOARGS | METH_O)) {
 PyCFunction meth = PyCFunction_GET_FUNCTION(func);
 PyObject *self = PyCFunction_GET_SELF(func);
 if (flags & METH_NOARGS && na == 0) {
 C_TRACE(x, (*meth)(self,NULL));
 x = _Py_CheckFunctionResult(func, x, NULL);
 }
The warning in GCC shouldn't probably have been enabled at all in `-Wall -Wextra` because the cast is explicit. However, it is somewhat true.
However, the correct way to fix would be to have the METH_NOARGS case cast the function to the right prototype. There exists lots of existing code that *is* going to break too. 
Perhaps PyCFunction should declare no prototype, i.e. empty parentheses, for backwards compatibility:
 typedef PyObject *(*PyCFunction)();
and deprecate it; start using a new typedef for it - and then add proper casts in every place that call a function.