WebSocketProtocolError is no longer exposed

[Harmon758]

The changelog for 8.0 says:

Version 8.0 renames the WebSocketProtocolError exception to :exc:ProtocolError .

A WebSocketProtocolError alias provides backwards compatibility.

However, WebSocketProtocolError is no longer exposed:

>>> import websockets
>>> websockets.WebSocketProtocolError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'websockets' has no attribute 'WebSocketProtocolError'

and this change breaks backwards compatibility.

[aaugustin]

Thanks, I'll make a bugfix release with this fix.