Make __iter__ and __len__ of DiGraphPredecessorsMapping networkx-compatible
Make __iter__ and __len__ of DiGraphPredecessorsMapping networkx-compatible

Same as for DiGraphSuccessorsMapping lately, it will iterate over all
available nodes, even if they don't have any edges leading to them.

I need a test that will fail with the old way of doing it and succeed
with the new.

LogicalDash/gorm