I've now written this up as a PEP: https://github.com/python/peps/blob/master/pep-0538.txt

The latest attached patch implements the specific design proposed in the PEP. Relative to the last Fedora specific patch, this tweaks the warning message wording slightly, and only emits the library level warning when PYTHONALLOWCLOCALE is set:

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$ LANG=C ./python -c "import sys; print(sys.getfilesystemencoding())"
Python detected LC_CTYPE=C, forcing LC_ALL & LANG to C.UTF-8 (set PYTHONALLOWCLOCALE to disable this locale coercion behaviour).
utf-8


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$ PYTHONALLOWCLOCALE=1 LANG=C ./python -c "import sys; print(sys.getfilesystemencoding())"
Py_Initialize detected LC_CTYPE=C, which limits Unicode compatibility. Some libraries and operating system interfaces may not work correctly. Set `PYTHONALLOWCLOCALE=1 LC_CTYPE=C` to configure a similar environment when running Python directly.
ascii