Draft: Add plpython3 package to haf image, needed for haf block explorer

This just adds the plpython3 package, allowing stored procedures to be written in python. Increases the image size by ~300k (0.02%).

Edit: @bwrona pointed out that haf block explorer no longer requires (as of sometime in the last week), so none of the core apps require plpython3 and we may not want this. There are conflicting opinions on whether we should leave plpython3 out of the image, or add it to make it easier for third-party apps want to use it, so I'm marking this as a draft until that's decided.