[merge][#277048] don't use iteritems on registries

[Andrew Bennetts]

Martin Pool wrote:
> Python doesn't like changing a dictionary while you're iterating it,
> and it seems, per <https://bugs.edge.launchpad.net/bzr/+bug/277048>
> that this can happen on the transport registry.  I'm not sure in
> precisely which case it would change the registry, but it probably
> depends on dictionary iteration order, and I suspect on us discovering
> that some dynamic dependencies are not there.  At any rate this patch
> seems like a safe fix for it.

bb:approve

-Andrew.