Partially specializing default plugin hooks

[JukkaL]

@gvanrossum said in #3534 (comment):

So a user plugin can override the default plugin. Should a hook returned by a user plugin be able to fall back dynamically on the default plugin's hook for the same function? E.g. suppose to add a special case for open() I would have to copy the default open() handler and modify it. It would be nice if there was some kind of "super call" mechanism for this.