Use package_data instead of MANIFEST.in

MANIFEST.in wasn't actually working, and package_data is
clearer

Author: yuvipanda

MANIFEST.in

@@ -18,5 +18,8 @@
             'shiny = jupyter_rsession_proxy:setup_shiny'
         ]
     },
+    package_data={
+        'jupyter_rsession_proxy': ['icons/*'],
+    },
     include_package_data=True
 )