diff --git a/oslo_concurrency/processutils.py b/oslo_concurrency/processutils.py
index 3ccbced..53a2d70 100644
--- a/oslo_concurrency/processutils.py
+++ b/oslo_concurrency/processutils.py
@@ -575,9 +575,9 @@ def ssh_execute(ssh, cmd, process_input=None,
             # to decode it, or an Unicode string otherwise. Encode to the
             # default encoding (ASCII) because mask_password() decodes from
             # the same encoding.
-            if isinstance(stdout, unicode):
+            if isinstance(stdout, six.text_type):
                 stdout = stdout.encode()
-            if isinstance(stderr, unicode):
+            if isinstance(stderr, six.text_type):
                 stderr = stderr.encode()
         else:
             # fsencode() is the reverse operation of fsdecode()
diff --git a/tox.ini b/tox.ini
index d004f41..a3174e5 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14,6 +14,7 @@ commands =
     env TEST_EVENTLET=1 lockutils-wrapper python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =