Make sphinx_rtd_theme optional

setup.py

@@ -53,7 +53,6 @@ requires = [
     'snowballstemmer>=1.1',
     'babel>=1.3,!=2.0',
     'alabaster>=0.7,<0.8',
-    'sphinx_rtd_theme>=0.1,<2.0',
 ]
 extras_require = {
     # Environment Marker works for wheel 0.24 or later