I don't think it's reasonable not to support multiple interpreters in a
single process - they're quite widely used by mod_python and mod_wsgi,
and probably by others.  I'm not sure whether that's a problem here or
not, though.

If we need to allow function annotations to be arbitrary PyObjects,
these PyObject pointers can't (in general) refer to statically allocated
python objects, so some extension modules will have to allocate them in
the module initialisation function (and presumably deallocate them again
when the module is unloaded).

I would have thought that any such PyObjects are going to be valid only
from within a single interpreter.  Perhaps I'm wrong.

Certainly it would be unpleasant if a change to one of the objects in
one interpreter was reflected in other interpreters, but if that didn't
risk causing a crash due to the memory allocation going wrong, or
something equally nasty, it might be acceptable.