Issue #24848: Fixed bugs in UTF-7 decoding of misformed data:

1. Non-ASCII bytes were accepted after shift sequence.
2. A low surrogate could be emitted in case of error in high surrogate.
3. In some circumstances the '\xfd' character was produced instead of the
replacement character '\ufffd' (due to a bug in _PyUnicodeWriter).

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

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

diff --cc Misc/NEWS
Simple merge

diff --cc Objects/unicodeobject.c
Simple merge