There was code to support set literals (on Python 2.7 and newer), but it
was buggy: SAFE_NODES.union() doesn't modify SAFE_NODES in place,
instead it returns a new set object that is then silently discarded.
I added a unit test and fixed the code. I also changed the version
check to use sys.version_tuple instead of a string comparison, for
consistency with the subsequent Python 3.4 version check that I added in
the previous commit.
