switch to py3 on docs job (#4230)

* switch to py3 on docs job

* somehow unicode isn't a word
diff --git a/tox.ini b/tox.ini
index 6dfe1c7..a1e97c5 100644
--- a/tox.ini
+++ b/tox.ini
@@ -39,7 +39,7 @@
 extras =
     docs
     docstest
-basepython = python2.7
+basepython = python3
 commands =
     sphinx-build -j4 -T -W -b html -d {envtmpdir}/doctrees docs docs/_build/html
     sphinx-build -j4 -T -W -b latex -d {envtmpdir}/doctrees docs docs/_build/latex