I think there is reason that CFunctionObjects are immutable: single 
CFunctionObject is shared by mutiple Python interpreters, so any change 
of CFunctionObject would affect other Python interpreters. Is that 
right?

If it should be immutable, then we should use something like static 
array to assign annotations to CFunctionObject, and the value also 
should be immutable, that means the value couldn't be abitrary 
PyObject. (by value I mean the value of every __annotations__ dict 
items.)

For SWIG, there's a way to bypass the Python side proxy, eg. for a 
simple C function, in the shadow module we directly 
let 'func=_cmod.func', where _cmod is the C DLL module. So the 
annotation information would be lost if we can't directly assign 
annotation to C function.