docs: Pin sphinx 1.7

The build currently fails on 1.8 due to https://bugs.python.org/issue34700

.travis.yml

@@ -47,7 +47,7 @@ install:
     # Ideally we'd run the docs/lint stuff on the latest Python
     # version supported. But Python 3.7 requires slower-to-start VMs,
     # so we run it on 3.6 to minimize total CI run time.
-    - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then travis_retry pip install sphinx sphinx_rtd_theme; fi
+    - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then travis_retry pip install -r docs/requirements.txt; fi
     - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then travis_retry pip install flake8 mypy; fi
     # On travis the extension should always be built
     - if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then export TORNADO_EXTENSION=1; fi

docs/requirements.txt

@@ -1 +1,4 @@
+# https://bugs.python.org/issue34700
+sphinx<1.8.0
+sphinx_rtd_theme
 Twisted

tox.ini

@@ -61,8 +61,7 @@ deps =
      {py35,py36,py37}-full: twisted
      py3: twisted
      {py3,py35,py36,py37}-full: pycares
-     sphinx: sphinx
-     sphinx: sphinx_rtd_theme
+     sphinx: -r{toxinidir}/docs/requirements.txt
      lint: flake8
 
 setenv =