add doctest by default

tox.ini

@@ -2,6 +2,7 @@
 envlist=
     py{35,36,py3}-core
     lint
+    doctest
 
 [isort]
 combine_as_imports=True
@@ -22,11 +23,14 @@ ignore=
 usedevelop=True
 commands=
     core: py.test {posargs:tests/core}
+    doctest: make -C {toxinidir}/docs doctest
 basepython =
     py35: python3.5
     py36: python3.6
     pypy3: pypy3
-extras=test
+extras=
+    test
+    doctest: doc
 
 [testenv:lint]
 basepython=python