Fix _PyOS_GetOpt() error message for non-ASCII characters

[vstinner]

Replace %c with %lc.

[vstinner]

I didn't write tests since assert_python_failure() returns bytes and it's not easy to get the right encoding. On Windows, it seems like print("%lc") uses the locale encoding which can be "cp1252" for example, it's not UTF-8. The strange thing is that on Windows, python -é displays the error message correctly (I tested cmd and powershell terminals). If a test is added later, I would suggest using locale.getencoding() to decode stderr.