Revert "try just python for report,codecov"

This reverts commit ea33016.

Author: altendky

tox.ini

@@ -15,8 +15,7 @@ basepython =
     py36: {env:TOXPYTHON:python3.6}
     py37: {env:TOXPYTHON:python3.7}
     py38: {env:TOXPYTHON:python3.8}
-    {bootstrap,clean,check}: {env:TOXPYTHON:python3}
-    {report,codecov}: {env:TOXPYTHON:python}
+    {bootstrap,clean,check,report,codecov}: {env:TOXPYTHON:python3}
 setenv =
     PYTHONPATH={toxinidir}/tests
     PYTHONUNBUFFERED=yes