diff --git a/acme/acme/client.py b/acme/acme/client.py
index 1fbd9ca5b..2576ed11a 100644
--- a/acme/acme/client.py
+++ b/acme/acme/client.py
@@ -8,7 +8,7 @@ from six.moves import http_client  # pylint: disable=import-error
 
 import OpenSSL
 import requests
-import six
+import sys
 import werkzeug
 
 from acme import errors
@@ -19,8 +19,9 @@ from acme import messages
 
 logger = logging.getLogger(__name__)
 
+# Python does not validate certificates by default before version 2.7.9
 # https://urllib3.readthedocs.org/en/latest/security.html#insecureplatformwarning
-if six.PY2:
+if sys.version_info < (2, 7, 9):  # pragma: no cover
     requests.packages.urllib3.contrib.pyopenssl.inject_into_urllib3()