py/objfun: Support fun.__globals__ attribute.

This returns a reference to the globals dict associated with the function,
ie the global scope that the function was defined in.  This attribute is
read-only but the dict itself is modifiable, per CPython behaviour.

Signed-off-by: Damien George <damien@micropython.org>

py/objfun.c

@@ -355,6 +355,10 @@ void mp_obj_fun_bc_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     if (attr == MP_QSTR___name__) {
         dest[0] = MP_OBJ_NEW_QSTR(mp_obj_fun_get_name(self_in));
     }
+    if (attr == MP_QSTR___globals__) {
+        mp_obj_fun_bc_t *self = MP_OBJ_TO_PTR(self_in);
+        dest[0] = MP_OBJ_FROM_PTR(self->globals);
+    }
 }
 #endif