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.
