bpo-29240: readline now ignores the UTF-8 Mode (#5145)

Add new fuctions ignoring the UTF-8 mode:

* _Py_DecodeCurrentLocale()
* _Py_EncodeCurrentLocale()
* _PyUnicode_DecodeCurrentLocaleAndSize()
* _PyUnicode_EncodeCurrentLocale()

Modify the readline module to use these functions.

Re-enable test_readline.test_nonascii().