sys.__stdin__ can be None

[minrk]

When the FD is not valid.

Technically, I think the same is possible for all std streams.

When this is the case, the block raises an AttributeError, which was not among the caught exceptions.

cf jupyterhub/jupyterhub#1108

[minrk]

This could also have been fixed by adding AttributeError to the exception list on L70. I figured that wouldn't be desirable since it would catch AttributeErrors due to potential future typos elsewhere, as well.

[takluyver]

Thanks Min, this looks reasonable to me. I'll give it a day or two in case @jquast or @MountainRider want to have a look.

[MountainRider]

I don't see any problems with this change.