py: Make mp_obj_get_type() return a const ptr to mp_obj_type_t.

Most types are in rodata/ROM, and mp_obj_base_t.type is a constant pointer,
so enforce this const-ness throughout the code base.  If a type ever needs
to be modified (eg a user type) then a simple cast can be used.

py/builtinhelp.c

@@ -134,7 +134,7 @@ STATIC void mp_help_print_obj(const mp_obj_t obj) {
     }
     #endif
 
-    mp_obj_type_t *type = mp_obj_get_type(obj);
+    const mp_obj_type_t *type = mp_obj_get_type(obj);
 
     // try to print something sensible about the given object
     mp_print_str(MP_PYTHON_PRINTER, "object ");