Add fake root to RST-dep tree

It is common practice in dependency parsing to add an artificial root node.
This root node governs the (actual) root(s) of the dependency structure.
It is usually placed at the beginning of the input sequence.

This artificial root node cannot have incoming dependencies, which should solve root-related downstream issues, e.g. in attelo's MST decoder.

Oh! Python 3 (now iterator).  Yes, Python 3 nails it.  I should probably stop using map and filter to avoid the confusion.  Alright, I'll be a good boy and just go with a standard pylintrc