Unwind `MultipleErrors` to have a plain list of `Invalid`

        :type errors: list[Invalid|MultipleInvalid]
        :rtype: list[Invalid]

def flatten(cls, errors):
        """ Unwind `MultipleErrors` to have a plain list of `Invalid`

        :type errors: list[Invalid|MultipleInvalid]
        :rtype: list[Invalid]
        """
        ers = []
        for e in errors:
            if isinstance(e, MultipleInvalid):
                ers.extend(cls.flatten(e.errors))
            else:
                ers.append(e)
        return ers