Use the new Unicode API

* Replace PyUnicode_FromUnicode(NULL, 0) by PyUnicode_New(0, 0)
 * Replce PyUnicode_FromUnicode(str, len) by PyUnicode_FromWideChar(str, len)
 * Replace Py_UNICODE by wchar_t
 * posix_putenv() uses PyUnicode_FromFormat() to create the string, instead
   of PyUnicode_FromUnicode() + _snwprintf()