only flush stream in dispatch_shell if possible

[Chilipp]

When using an inprocess kernel, the kernel client calls the dispatch_shell method with an ipykernel.inprocess.socker.DummySocket

ipykernel/ipykernel/inprocess/client.py

Line 170 in b81632e

kernel.dispatch_shell(stream, msg_parts)

This method, however, expects the given stream to have a flush method

ipykernel/ipykernel/kernelbase.py

Line 286 in b81632e

stream.flush(zmq.POLLOUT)

which is not True, for the DummySocket. This PR proposes to accept failures for the latter by replacing this line with

try:
    stream.flush(zmq.POLLOUT)
except AttributeError:
    pass

[minrk]

Thanks for the PR! I think it's a valid assumption that the stream provide the stream API. Maybe the better fix is to add a no-op flush method to DummySocket so that it complies with the stream API?

[Chilipp]

You're right @minrk, I fully agree. It revert the previous implementation (14a2c7a) and added a dummy flush method (c9ec713)

[minrk]

Thanks!