PyUnicode_AsEncodedString() contains a special path for the file system encoding. I don't think that it is still needed, but I don't know how to check that.
/* During bootstrap, we may need to find the encodings
package, to load the file system encoding, and require the
file system encoding in order to load the encodings
package.
Break out of this dependency by assuming that the path to
the encodings module is ASCII-only. XXX could try wcstombs
instead, if the file system encoding is the locale's
encoding. */
else if (Py_FileSystemDefaultEncoding &&
strcmp(encoding, Py_FileSystemDefaultEncoding) == 0 &&
!PyThreadState_GET()->interp->codecs_initialized)
return PyUnicode_EncodeASCII(PyUnicode_AS_UNICODE(unicode),
PyUnicode_GET_SIZE(unicode),
errors);