param signature_filename: The pathname to the file containing the
                           signature.
:param data_filename: The pathname to the file containing the
                      signed data.
:param keystore: The path to a directory which contains the keys
                 used in verification. If not specified, the
                 instance's ``gpg_home`` attribute is used instead.
:return: True if the signature was verified, else False.
z0verification unavailable because gpg unavailable)