remove simplegeneric from external

make it a dependency

setup.py

@@ -271,6 +271,7 @@ extras_require['nbconvert'].extend(extras_require['nbformat'])
 install_requires = [
     'decorator',
     'path.py', # required by pickleshare, remove when pickleshare is added here
+    'simplegeneric>0.8',
 ]
 
 # add platform-specific dependencies