I'm sorry for the trouble. It should appear in the contributions manager again soon. Until it does, you should be able to download http://py.processing.org/3/PythonMode.zip and unzip it into the "modes" directory of your Processing "sketchbook" folder (which is different for each platform).
I'm sorry for the trouble. It should appear in the contributions manager again soon. Until it does, you should be able to download http://py.processing.org/3/PythonMode.zip and unzip it into the "modes" directory of your Processing "sketchbook" folder (which is different for each platform).