[gdb/python] Fix python selftest with python 3.11

With python 3.11 I noticed:
...
$ gdb -q -batch -ex "maint selftest python"
Running selftest python.
Self test failed: self-test failed at gdb/python/python.c:2246
Ran 1 unit tests, 1 failed
...

In more detail:
...
(gdb) p output
$5 = "Traceback (most recent call last):\n  File \"<string>\", line 0, \
  in <module>\nKeyboardInterrupt\n"
(gdb) p ref_output
$6 = "Traceback (most recent call last):\n  File \"<string>\", line 1, \
  in <module>\nKeyboardInterrupt\n"
...

Fix this by also allowing line number 0.

Tested on x86_64-linux.

This should hopefully fix buildbot builder gdb-rawhide-x86_64.