Message 258796 - Python tracker

I pushed my latest patch with minor changes in comments.

I will wait for buildbots before backporting the change to Python 2.7 and 3.5. For the backport, I will probably remove the optimization on frozenset since it's only useful for AST optimizers (the optimization is a new feature, I considered that it was worth it to add it Python 3.6 as part of my work on the PEP 511).