Issue #28426: Deprecated undocumented functions PyUnicode_AsEncodedObject(),

PyUnicode_AsDecodedObject(), PyUnicode_AsDecodedUnicode() and
PyUnicode_AsEncodedUnicode().

diff --cc Doc/whatsnew/3.6.rst
Simple merge

diff --cc Misc/NEWS
Simple merge

diff --cc Objects/unicodeobject.c
Simple merge