RTD uses 3.5, so should we

We split up building of docs and doctest because RTD doesn't allow newer than
3.5 but we need up to date Python versions for our doctests.

.travis.yml

@@ -44,9 +44,12 @@ matrix:
       env: TOXENV=py37
 
     # Docs
-    - python: "3.7"
+    - python: "3.5"
       stage: docs
       env: TOXENV=docs
+    - python: "3.7"
+      stage: doctest
+      env: TOXENV=doctest
     - python: "3.7"
       env: TOXENV=pypi-description
     - python: "3.7"

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = typing,lint,py27,py34,py35,py36,py37,pypy,pypy3,manifest,docs,pypi-description,changelog,coverage-report
+envlist = typing,lint,py27,py34,py35,py36,py37,pypy,pypy3,manifest,docs,doctest,pypi-description,changelog,coverage-report
 
 
 [testenv]
@@ -40,16 +40,21 @@ basepython = python3.7
 skip_install = true
 deps = pre-commit
 passenv = HOMEPATH  # needed on Windows
-commands = pre-commit run --all-files --verbose
+commands = pre-commit run --all-files
 
 
 [testenv:docs]
+# RTD only allows for 3.5
+basepython = python3.5
+extras = docs
+commands = sphinx-build -W -b html -d {envtmpdir}/doctrees docs docs/_build/html
+
+
+[testenv:doctest]
+# We need up to date Python versions for our doctests.
 basepython = python3.7
-setenv =
-    PYTHONHASHSEED = 0
 extras = docs
 commands =
-    sphinx-build -W -b html -d {envtmpdir}/doctrees docs docs/_build/html
     sphinx-build -W -b doctest -d {envtmpdir}/doctrees docs docs/_build/html
     python -m doctest README.rst