Run coverage from the outside so it sees everything
diff --git a/tox.ini b/tox.ini
index 4d17ebe..88e7421 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,9 +3,12 @@
 
 [testenv]
 deps =
-    pytest-cov
+    pytest
+    coverage
     pretend
-commands = py.test --cov=cryptography/ --cov=tests/
+commands =
+    coverage run --source=cryptography/,tests/ -m pytest
+    coverage report
 
 [testenv:docs]
 deps = sphinx