End support for Python 2

As planned and advertized, yamllint drops support for Python 2 on 2021.

tests/test_cli.py

@@ -14,10 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-try:
-    from cStringIO import StringIO
-except ImportError:
-    from io import StringIO
+from io import StringIO
 import fcntl
 import locale
 import os
@@ -71,11 +68,6 @@ class CommandLineTestCase(unittest.TestCase):
     def setUpClass(cls):
         super(CommandLineTestCase, cls).setUpClass()
 
-        # https://docs.python.org/3/library/unittest.html#deprecated-aliases
-        if sys.version_info < (3, 2):
-            cls.assertRegex = cls.assertRegexpMatches
-            cls.assertRaisesRegex = cls.assertRaisesRegexp
-
         cls.wd = build_temp_workspace({
             # .yaml file at root
             'a.yaml': '---\n'