Restore usedevelop=True now that pip 19.1.1 restored the old behavior

Fix pytest-dev#5167

tox.ini

@@ -73,8 +73,7 @@ commands = pre-commit run --all-files --show-diff-on-failure
 
 [testenv:docs]
 basepython = python3
-# broken due to pip 19.1 (#5167)
-# usedevelop = True
+usedevelop = True
 changedir = doc/en
 deps = -r{toxinidir}/doc/en/requirements.txt
 
@@ -128,8 +127,7 @@ commands =
 [testenv:release]
 decription = do a release, required posarg of the version number
 basepython = python3.6
-# broken due to pip 19.1 (#5167)
-# usedevelop = True
+usedevelop = True
 passenv = *
 deps =
     colorama