Should just return self._when attribute.

I need it for testing purposes.
Without the method there is no possibility to figure out scheduled wakeup time.
An alternative is waiting for callback but the approach increases unittest execution time and not very reliable.

I don't like looking into `handler._when` private property, uvloop does not expose it.

Ned, is it possible to include the feature into Python 3.7? 
The change is safe and tiny, no backward compatibility problems etc.