fix devpi test for doctesting env
fix devpi test for doctesting env

due to a devpi bug, we always get a sdist install which in turn triggers
the pytest issue #2042 / #726

going for pyargs and a changed folder, it should no longer happen
(and will be tested firther after rebasing the release branch)

jaraco/pytest,pytest-dev/pytest,markshao/pytest,skylarjhdownes/pytest,flub/pytest,RonnyPfannschmidt/pytest,txomon/pytest,pfctdayelise/pytest,tgoodlet/pytest,MichaelAquilina/pytest,alfredodeza/pytest,ddboline/pytest,davidszotten/pytest,The-Compiler/pytest,tomviner/pytest,vmalloc/dessert,tareqalayan/pytest,rmfitzpatrick/pytest,nicoddemus/pytest,hackebrot/pytest,Akasurde/pytest,nicoddemus/pytest,etataurov/pytest,The-Compiler/pytest,tomviner/pytest,eli-b/pytest