TST: skip the test if a known problem with Python 2.6 is detected

As was original "discovered" while running tests on OSX with python2.6:
http://nipy.bic.berkeley.edu/builders/fail2ban-py2.7-osx-10.6_master/builds/6/steps/shell_2/logs/stdio
DOC: minor -- added a link to pull requests in CONTRIBUTING