It should be relatively easy to devise a new PyGILState API with support for multiple interpreters. We just need two new functions (similar to the two existing ones) taking a PyInterpreterState* parameter; a TLS key can be added to the PyInterpreterState structure (instead of the current global TLS key).

It will be up to the caller to know which interpreter they want to hook into when calling these functions (which is application-dependent and is normally well-defined, e.g. when calling a Python callback, you should call it with the interpreter which was in use when registering the callback (i.e. ``PyThreadState_Get()->interp``)).