py/objboundmeth: Support __func__ property as in CPython

This gives access to the function underlying the bound method.

Used in the converted CPython stdlib logging.Formatter class to handle
overrriding a default converter method bound to a class variable.
The method becomes bound when accessed from an instance of that class.

I didn't investigate why CircuitPython turns it into a bound method.

py/objboundmeth.c

@@ -92,6 +92,9 @@ STATIC void bound_meth_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     if (attr == MP_QSTR___name__) {
         mp_obj_bound_meth_t *o = MP_OBJ_TO_PTR(self_in);
         dest[0] = MP_OBJ_NEW_QSTR(mp_obj_fun_get_name(o->meth));
+    } else if (attr == MP_QSTR___func__) {
+        mp_obj_bound_meth_t *o = MP_OBJ_TO_PTR(self_in);
+        dest[0] = o->meth;
     }
 }
 #endif