make sure tests pass without a manifest.in (tox builds an sdist)

tox.ini

@@ -13,6 +13,7 @@ commands =
 deps =
     mock >= 0.5.0
     meld3 >= 0.6.5
+usedevelop = true
 
 [testenv:cover]
 basepython =
@@ -25,3 +26,4 @@ deps =
     nose
     coverage
     nosexcover
+usedevelop = true