block sphinx 1.8.0 since it fails our docs job (#4469)

diff --git a/setup.py b/setup.py
index 0c793e3..5c8c584 100644
--- a/setup.py
+++ b/setup.py
@@ -297,7 +297,7 @@
 
         "test": test_requirements,
         "docs": [
-            "sphinx >= 1.6.5",
+            "sphinx >= 1.6.5,!=1.8.0",
             "sphinx_rtd_theme",
         ],
         "docstest": [