While going for the full locale setting may be a good option,
perhaps just focusing on the FS encoding for now is a better
way forward (and also more in line with the ticket title).

So essentially go for the PEP 529 approach on Unix as well
(except that we use 'ascii' as fallback in legacy mode):

https://www.python.org/dev/peps/pep-0529/

The PEP also includes a section on affected modules, which we
could double check (even though the term "FS encoding" implies
that only file system relevant APIs are touched by such a change,
the encoding is used in several other places as well):

https://www.python.org/dev/peps/pep-0529/#id14

For Windows, a couple of modules such as pwd and nis are not
used, so those may need some extra attention.