ax.AXKeyValueMessage.getSingle will never raise KeyError

[evgeni]

https://github.com/openid/python-openid/blob/master/openid/extensions/ax.py#L546 says:
@raises KeyError: If the attribute was not sent in this response
However, KeyError is never raised explicitely and
values = self.data.get(type_uri) will return None when type_uri isn't found in data.

Either documentation or code should be edited :)