fix: sphinx crash with 1.8 due to python 3 dependencies

setup.py

@@ -229,7 +229,7 @@ def setup_package():
             'matplotlib<3.0',
             'docutils>=0.3',
             'six>=1.10',
-            'Sphinx<=1.8',
+            'Sphinx<1.8',
             'sphinx_rtd_theme',
             'm2r<=0.2.1',
             # 'sphinx_bootstrap_theme',