Merge pull request #1285 from tseaver/workaround-tox-2.3-regression

Workaround tox 2.3.0 regression.

Author: tseaver

tox.ini

@@ -54,7 +54,7 @@ passenv = {[testenv:system-tests]passenv} SPHINX_RELEASE READTHEDOCS LOCAL_RTD
 
 [testenv:docs-rtd]
 setenv =
-    {[testenv]setenv}
+    PYTHONPATH = {toxinidir}/_testing
     LOCAL_RTD = True
 basepython = {[testenv:docs]basepython}
 commands =