Issue #16836: Enable IPv6 support even if IPv6 is disabled on the build host.
Fix out of bound read in UTF-32 decoder on "narrow Unicode" builds.
Issue #11461: Fix the incremental UTF-16 decoder. Original patch by
Amaury Forgeot d'Arc. Added tests for partial decoding of non-BMP
characters.
Issue #16491: IDLE now prints chained exception tracebacks.