Issue #13612: handle unknown encodings without a buffer overflow.

This affects pyexpat and _elementtree. PyExpat_CAPI now exposes a new
function - DefaultUnknownEncodingHandler.

Based on a patch by Serhiy Storchaka.

diff --cc Lib/test/test_xml_etree.py
Simple merge

diff --cc Modules/_elementtree.c
Simple merge