fix devpi test for doctesting env

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

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

tox.ini

@@ -118,11 +118,13 @@ commands=
 basepython = python
 usedevelop=True
 skipsdist=True
+# ensure the given pyargs cant mean anytrhing else
+changedir=doc/
 deps=
     PyYAML
 commands=
-    pytest -rfsxX doc/en
-    pytest --doctest-modules {toxinidir}/_pytest
+    pytest -rfsxX en
+    pytest --doctest-modules --pyargs _pytest
 
 [testenv:regen]
 changedir=doc/en