Fix invalid function cast warnings with gcc 8
for method conventions different from METH_NOARGS, METH_O and
METH_VARARGS in Argument Clinic generated code.
The function '_PyArg_ParseStack()' and
'_PyArg_UnpackStack' were failing (with error
"XXX() takes Y argument (Z given)") before
the function '_PyArg_NoStackKeywords()' was called.
Thus, the latter did not raise its more meaningful
error : "XXX() takes no keyword arguments".
Issue #29286. Run Argument Clinic to get the new faster METH_FASTCALL calling
convention for functions using "boring" positional arguments.
Manually fix _elementtree: _elementtree_XMLParser_doctype() must remain
consistent with the clinic code.
This matches the usage of ZipFile and BufferedWriter. This still requires
return values to be bytes() objects.
Also document and test that the write() methods should only access their
argument before they return.
Serhiy Storchaka
Sylvain
orenmn
Victor Stinner
Martin Panter