bpo-35883: Py_DecodeLocale() escapes invalid Unicode characters (GH-24843)

Python no longer fails at startup with a fatal error if a command
line argument contains an invalid Unicode character.

The Py_DecodeLocale() function now escapes byte sequences which would
be decoded as Unicode characters outside the [U+0000; U+10ffff]
range.

Use MAX_UNICODE constant in unicodeobject.c.