pin sphinx version to avoid test breakage (#3544)

diff --git a/setup.py b/setup.py
index 7c23b00..e5e6949 100644
--- a/setup.py
+++ b/setup.py
@@ -314,7 +314,7 @@
             "doc8",
             "pyenchant",
             "readme_renderer >= 16.0",
-            "sphinx",
+            "sphinx != 1.6.1",
             "sphinx_rtd_theme",
             "sphinxcontrib-spelling",
         ],