The workaround would be to unlink the file first, and then try to
create it with O_EXCL. You have a short window where there's no file,
but that shouldn't be a problem in this specific case, and it would
work on Windows.

As for issue #17222, well, many applications use temporary files and
rename (e.g. most web browsers), so I'd be tempted to say "don't do
it".
Of course, I would feel kinda bad if Python broke Debian's builders (I
don't care about Gentoo though ;-)

Its funny how an seemingly harmless change can introduce nasty regressions...