Issue #15984: Merge PyUnicode doc from 3.5

diff --cc Doc/c-api/exceptions.rst
Simple merge

diff --cc Doc/library/logging.handlers.rst
Simple merge

diff --cc Doc/library/os.rst
Simple merge

diff --cc Include/bytesobject.h
Simple merge

diff --cc Include/unicodeobject.h
Simple merge

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

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

diff --cc Misc/NEWS
Simple merge

diff --cc Modules/_heapqmodule.c
Simple merge

diff --cc Modules/_pickle.c
Simple merge

diff --cc Modules/_tracemalloc.c
Simple merge

diff --cc Objects/codeobject.c
Simple merge

diff --cc Objects/unicodeobject.c
Simple merge