remove unused six imports

acme/acme/messages_test.py

@@ -3,7 +3,6 @@ import unittest
 
 import josepy as jose
 import mock
-import six
 
 from acme import challenges
 from acme import test_util

acme/acme/standalone_test.py

@@ -6,7 +6,6 @@ import threading
 import tempfile
 import unittest
 
-import six
 from six.moves import http_client  # pylint: disable=import-error
 from six.moves import queue  # pylint: disable=import-error
 from six.moves import socketserver  # type: ignore  # pylint: disable=import-error

acme/acme/util_test.py

@@ -1,8 +1,6 @@
 """Tests for acme.util."""
 import unittest
 
-import six
-
 
 class MapKeysTest(unittest.TestCase):
     """Tests for acme.util.map_keys."""

certbot-dns-rfc2136/certbot_dns_rfc2136/dns_rfc2136_test.py

@@ -7,7 +7,6 @@ import dns.flags
 import dns.rcode
 import dns.tsig
 import mock
-import six
 
 from certbot import errors
 from certbot.plugins import dns_test_common