<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_config_fname():
 """Helper for the vispy config file"""

    directory = _get_vispy_app_dir()
    if directory is None:
        return None
    fname = op.join(directory, 'vispy.json')
    if os.environ.get('_VISPY_CONFIG_TESTING', None) is not None:
        fname = op.join(_TempDir(), 'vispy.json')
    return fname