[gdb/testsuite] Fix gdb.python/py-warning.exp with python 3.6

[gdb/testsuite] Fix gdb.python/py-warning.exp with python 3.6

On openSUSE Tumbleweed (with python 3.13), I get:
...
(gdb) PASS: gdb.python/py-warning.exp: python gdb.warning("")
python gdb.warning()^M
Python Exception <class 'TypeError'>: \
  function missing required argument 'text' (pos 1)^M
Error occurred in Python: function missing required argument 'text' (pos 1)^M
(gdb) PASS: gdb.python/py-warning.exp: python gdb.warning()
...

But on openSUSE Leap 15.6 (with python 3.6), I get instead:
...
(gdb) PASS: gdb.python/py-warning.exp: python gdb.warning("")
python gdb.warning()^M
Python Exception <class 'TypeError'>: \
  Required argument 'text' (pos 1) not found^M
Error occurred in Python: Required argument 'text' (pos 1) not found^M
(gdb) FAIL: gdb.python/py-warning.exp: python gdb.warning()
...

Fix this by updating the regexp.

Tested on x86_64-linux.

PR testsuite/33104
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33104