Use django.utils.six for PY2/PY3 compatibility
Use input from django.utils.six.moves, remove cms.utils.compat.input module
Use django.utils.six.moves for StringIO, one more file from CMS is gone
Updated CHANGELOG.txt file to fix release information
Merge pull request #3289 from mmarzantowicz/feature/PY2_PY3_support_cleanup

Use django.utils.six for PY2/PY3 compatibility