FTR I thought the consensus was not to backport these fixes unless there was a demonstrated problem: <https://bugs.python.org/issue1621#msg144499>, though personally, I would be in favour of backporting in many cases.

Regarding str.join() in unicode.c, see also my unicode.patch in that bug. I included a test case that works on 32-bit platforms, which you may use. (From memory, you probably need to disable -fwrapv and maybe use -ftrapv or the undefined behaviour sanitizer to make the test fail.)