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/common.py

@@ -20,7 +20,7 @@ import sys
 try:
     assert sys.version_info >= (2, 7)
     import unittest
-except:
+except AssertionError:
     import unittest2 as unittest
 
 import yaml

tests/test_cli.py

@@ -27,7 +27,7 @@ import sys
 try:
     assert sys.version_info >= (2, 7)
     import unittest
-except:
+except AssertionError:
     import unittest2 as unittest
 
 from yamllint import cli

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

tests/test_linter.py

@@ -19,7 +19,7 @@ import sys
 try:
     assert sys.version_info >= (2, 7)
     import unittest
-except:
+except AssertionError:
     import unittest2 as unittest
 
 from yamllint.config import YamlLintConfig

tests/test_module.py

@@ -22,7 +22,7 @@ import sys
 try:
     assert sys.version_info >= (2, 7)
     import unittest
-except:
+except AssertionError:
     import unittest2 as unittest
 
 

tests/test_parser.py

@@ -18,7 +18,7 @@ import sys
 try:
     assert sys.version_info >= (2, 7)
     import unittest
-except:
+except AssertionError:
     import unittest2 as unittest
 
 import yaml