Fixed the declaration of some public API functions. PyArg_VaParse() and
PyArg_VaParseTupleAndKeywords() were not available in limited API.
PyArg_ValidateKeywordArguments(), PyArg_UnpackTuple() and Py_BuildValue()
were not available in limited API of version < 3.3 when PY_SSIZE_T_CLEAN is
defined.
