Sign a file.

        :param filename: The pathname to the file to be signed.
        :param signer: The identifier of the signer of the file.
        :param sign_password: The passphrase for the signer's
                              private key used for signing.
        :param keystore: The path to a directory which contains the keys
                         used in signing. If not specified, the instance's
                         ``gpg_home`` attribute is used instead.
        :return: The absolute pathname of the file where the signature is
                 stored.

def sign_file(self, filename, signer, sign_password, keystore=None):
        """
        Sign a file.

        :param filename: The pathname to the file to be signed.
        :param signer: The identifier of the signer of the file.
        :param sign_password: The passphrase for the signer's
                              private key used for signing.
        :param keystore: The path to a directory which contains the keys
                         used in signing. If not specified, the instance's
                         ``gpg_home`` attribute is used instead.
        :return: The absolute pathname of the file where the signature is
                 stored.
        """
        cmd, sig_file = self.get_sign_command(filename, signer, sign_password,
                                              keystore)
        rc, stdout, stderr = self.run_command(cmd,
                                              sign_password.encode('utf-8'))
        if rc != 0:
            raise DistlibException('sign command failed with error '
                                   'code %s' % rc)
        return sig_file