./python -c "print('à')"
does not work on my Linux machine with latest py3k (r66303), certainly
because my terminal uses a latin-1 encoding: wcstombs will convert the
argument back to the terminal encoding, whereas PyRun_SimpleString
expects a UTF-8 string.

I join another patch, which propagates the wchar_t as far as possible,
and encodes it as utf-8; with test.

This also corrects the Windows case.