Skip py2 only test on py3

When running the tests with python3 setup.py test all the tests nose can
find are run.
This commit prevents that the py2-only test is run on py3

Relates to https://github.com/agoragames/chai/issues/22
Skip more tests failing on py3
Adjust unit-test for change in behavior between py2 and py3
Officially drop support for python 2.6
Merge pull request #23 from pypingou/fix_tests

Fix tests for python 2.7 and 3.3