The test cases for locale coercion *not* triggering still assume that bpo-19977, using surrogateescape on the standard streams in the POSIX locale, has been implemented (since that was implemented in Python 3.5).

Hence the various test cases complaining that they found "ascii:strict" (Py 3.4 behaviour without bpo-19977) where they expected "ascii:surrogateescape" (the Py 3.5+ behaviour *with* bpo-19977).

To get a PEP 538 backport to work as intended on 3.4, you'd need to backport that earlier IO stream error handling change as well.