In short I'm not against making PickleBuffer optional but I'm not sure it makes a lot of sense.  Would you make memoryview optional because it's written in C?  Is anyone annoyed by the fact that something comes from _pickle.c.

(note that the pure Python Pickler class *does* work with PickleBuffer; so you don't lose any extensability or customizability by using PickleBuffer)

In any case, I won't do the work myself, so feel free to draft a PR so that we see the magnitude of the changes required.