gdb/testsuite: fix gdb.python/py-unwind.exp with python >= 3.11

Python 3.11 changed the AttributeError message - see commit
0cb765b2cec9 ("bpo-46730: Add more info to @property AttributeError
messages (GH-31311)").  Add the new message to the expectations.

Approved-By: Tom Tromey <tom@tromey.com>
Link: https://sourceware.org/pipermail/gdb-patches/2023-June/200433.html