Use importlib.machinery.all_suffixes if possible for open files test
Test for open files with travis
Support Python 2.6's lack of importlib

This change is made on a distro without python2.6 support, so good luck
Import importlib.machinery in open-files pytest plugin
Check for open files as part of all the base tests
Merge pull request #3096 from sargas/fix-python33-open-files

Fix --open-files test with Python 3.3 and newer