Use editable installs to measure code coverage correctly.

tox.ini

@@ -54,6 +54,7 @@ basepython =
     3.6: python3.6
     pypy: pypy
     flake8,flakeplus,apicheck,linkcheck,configcheck,pydocstyle,isort-check,bandit: python2.7
+usedevelop = True
 
 [testenv:apicheck]
 setenv =