diff --git a/tox.ini b/tox.ini
index 179745a..0f7e3a3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,5 +3,7 @@ isolated_build = true
 envlist = py27, py37, py38, py39, py310, pypy, pypy3
 
 [testenv]
-deps = py27,pypy: mock
+deps =
+  py27,pypy: mock
+  py27,pypy: contextlib2
 commands = python -m unittest discover -p *_test.py