> it is probably not possible to write a pure Python PickleBuffer

Fair enough

> a usable pure Python Pickler, but without support for the PickleBuffer class.

That makes sense. However, for third party packages (e.g. zodbpickle, pikl) wanting a pure Python pickle module to use as a basis, I think it would make more sense to use the CPython 3.7 branch as a basis. Adding a pickle._Pickler variant to the 3.8 stdlib that can't do protocol 5, just for third-parties imposes a maintenance burden on the core devs for zero benefit to anyone.

I propose closing this ticket as WONTFIX.