[docs] [issue2716] Document license under which audioop is used

Change by Terry J. Reedy <tjreedy@udel.edu>: ---------- versions: +Python 3.7, Python 3.8, Python 3.9 -Python 2.7, Python 3.4, Python 3.5 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue2716> _______________________________________