'Get the legal arguments for the setter associated with *attr*.
This is done by querying the docstring of the function *set_attr*
for a line that begins with ACCEPTS:
e.g., for a line linestyle, return
"[ ``\'-\'`` | ``\'--\'`` | ``\'-.\'`` | ``\':\'`` | ``\'steps\'`` | ``\'None\'``'

def get_valid_values(self, attr):

    name = (u'set_%s' % attr)
    if (not hasattr(self.o, name)):
        raise AttributeError((u'%s    has    no    function    %s' % (self.o, name)))
    func = getattr(self.o, name)
    docstring = func.__doc__
    if (docstring is None):
        return u'unknown'
    if docstring.startswith(u'alias    for    '):
        return None
    match = self._get_valid_values_regex.search(docstring)
    if (match is not None):
        return re.sub(u'\n    *', u'    ', match.group(1))
    return u'unknown'
