Tests: Made another test directly executable with Python3 and more expressive.
Do not warn about non-recursion into missing modules that are white listed.
Fix, yield from needs to preserve and restore exceptions like yield does.

* This solves a TODO, it was unclear if that was necessary or not.

* Clarified by adding a test that outputs what it really is.
CLeanup scons file

* Done with autoformat tool

* Also a few minor manual cleanups
Fix, when updating env["CXX"] for scons, need to unset env["CXXVERSION"]

* This corrects Issue#160, which is caused by checking g++ version as
  initially found by Scons and not the version of the compiler we changed
  to.

* Support hardening-wrapper of Debian too, it has single digits versions
  for gcc, remove those too.
Minor cleanup

* Better message for print nodes

* Discover "None" arguments for printed items sooner.
Fix, printed unicode values may not encode to strings.

* This was assuming all constants can be printed, which is far
  from true in the unicode case on Python2 at least.

* Could be triggered by printing strings from encoded source
  files.
Changelog updates for release
New stable release.
Merge branch 'release/0.5.6'
Merge tag '0.5.6' into develop
New release cycle.