Performs an authentication.

        Uses either the private token, or the email/password pair.

        The `user` attribute will hold a `gitlab.objects.CurrentUser` object on
        success.

def auth(self):
        """Performs an authentication.

        Uses either the private token, or the email/password pair.

        The `user` attribute will hold a `gitlab.objects.CurrentUser` object on
        success.
        """
        if self.private_token or self.oauth_token:
            self._token_auth()
        else:
            self._credentials_auth()