sphinx 1.6.3 is also no good (#3748)

diff --git a/setup.py b/setup.py
index 8bb591f..cc3b706 100644
--- a/setup.py
+++ b/setup.py
@@ -295,7 +295,7 @@
             "doc8",
             "pyenchant",
             "readme_renderer >= 16.0",
-            "sphinx != 1.6.1, != 1.6.2",
+            "sphinx != 1.6.1, != 1.6.2, != 1.6.3",
             "sphinx_rtd_theme",
             "sphinxcontrib-spelling",
         ],