style: Fix E722 errors reported by pycodestyle

Since a few days ago pycodestyle (formerly called pep8) has a new check:
E722 warning for bare except clauses.

Let's fix our code.

tests/test_config.py

@@ -24,7 +24,7 @@ import sys
 try:
     assert sys.version_info >= (2, 7)
     import unittest
-except:
+except AssertionError:
     import unittest2 as unittest
 
 from yamllint import cli