FYI, I've already started some of this work:

* #36737 warnings
* #36854 gc
* #33608 pending calls
* #10915 & #15751 gilstate

Other bits I'm planning on:
* the rest of the global "ceval" state
* the memory allocators
* the GIL

Note that, to make the GIL per-interpreter, we can't have any remaining runtime state shared by interpreters (unless it is protected by its own locks).