Add a user to an issue's watchers list.

        :param issue: ID or key of the issue affected
        :param watcher: username of the user to add to the watchers list

def add_watcher(self, issue, watcher):
        """Add a user to an issue's watchers list.

        :param issue: ID or key of the issue affected
        :param watcher: username of the user to add to the watchers list
        """
        url = self._get_url('issue/' + str(issue) + '/watchers')
        self._session.post(
            url, data=json.dumps(watcher))