release 4.60

• Pyro4.core.async() and proxy._pyroAsync() now return None, instead of the proxy object.
  This means you'll have to change your code that expects a proxy as return value, for instance by creating a
  copy of the proxy yourself first.
  This change was done to avoid subtle errors where older code still assumed it got a copy of the proxy,
  but since 4.57 that is no longer done and it is handed back the same proxy.
  By returning None now, at least the old code will now crash with a clear error, instead of silently continuing
  with the possibility of failing in weird ways later.