Fix build on Python 3.4

Author: mnaberez

tox.ini

@@ -4,6 +4,7 @@ envlist =
 
 [testenv]
 deps =
+    attrs < 21.1.0  # see https://github.com/python-attrs/attrs/pull/608
     pytest
     pexpect == 4.7.0  # see https://github.com/Supervisor/supervisor/issues/1327
     mock >= 0.5.0