'Returns certificate from URI.
:param str uri: URI of certificate
:returns: tuple of the form
(response, :class:`acme.jose.ComparableX509`)
:rtype: tuple'

def _get_cert(self, uri):

    content_type = DER_CONTENT_TYPE
    response = self.net.get(uri, headers={'Accept': content_type}, content_type=content_type)
    return (response, jose.ComparableX509(OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_ASN1, response.content)))
