disable smart quotes in sphinx to workaround a spellchecker issue (#3946)

This makes it so we don't have to pin sphinx while we wait for
sphinxcontrib-spelling to properly handle smart quotes
diff --git a/setup.py b/setup.py
index e69ffee..c5a42b2 100644
--- a/setup.py
+++ b/setup.py
@@ -290,7 +290,7 @@
             "doc8",
             "pyenchant >= 1.6.11",
             "readme_renderer >= 16.0",
-            "sphinx != 1.6.1, != 1.6.2, != 1.6.3, != 1.6.4",
+            "sphinx",
             "sphinx_rtd_theme",
             "sphinxcontrib-spelling",
         ],