Add missing sphinx dependency to setup.py

setup.py

@@ -112,13 +112,13 @@ setup(
         "requests==2.22.0",
         "scikit-learn==0.22.2.post1",
         "sharedmem==0.3.8",
+        "sphinx==1.8.5",
         "tabulate==0.8.6",
         "traitlets==4.3.3",
     ],
     extras_require={
         "docs": [
             "nbsphinx",
-            "sphinx==1.8.5",
         ],
         "test": [
             "coverage",