Right now, the legacy locale detection introduced in PEP 538 doesn't trigger for "LANG=POSIX" and "LC_CTYPE=POSIX" on macOS and other *BSD systems.

This is because we're looking specifically for "C" as the response from "setlocale(LC_CTYPE, NULL)", which works on Linux (where glibc reports "C" if you configured "POSIX"), but not on *BSD systems (where POSIX and C behave the same way, but are still reported as distinct locales).

As per Jakub Wilk's comments at https://mail.python.org/pipermail/python-dev/2017-December/151105.html, this isn't right: we should allow either string to be returned from setlocale, and consider both of them as indicating a legacy locale to be coerced to an explicitly UTF-8 based one if possible.