[gdb/testsuite] Fix gdb.python/py-format-string.exp with python 3.13

On fedora rawhide, with python 3.13, I run into:
...
(gdb) python print (gdb.parse_and_eval ('a_point_t').format_string (invalid=True))^M
Python Exception <class 'TypeError'>: \
  this function got an unexpected keyword argument 'invalid'^M
Error occurred in Python: \
  this function got an unexpected keyword argument 'invalid'^M
(gdb) FAIL: $exp: format_string: lang_c: test_all_common: test_invalid_args: \
  a_point_t with option invalid=True
...

A passing version with an older python version looks like:
...
(gdb) python print (gdb.parse_and_eval ('a_point_t').format_string (invalid=True))^M
Python Exception <class 'TypeError'>: \
  'invalid' is an invalid keyword argument for this function^M
Error occurred in Python: \
  'invalid' is an invalid keyword argument for this function^M
(gdb) PASS: $exp: format_string: lang_c: test_all_common: test_invalid_args: \
  a_point_t with option invalid=True
...

Fix this by accepting the updated error message.

Tested on aarch64-linux.

PR testsuite/31912
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31912

gdb/testsuite/gdb.python/py-format-string.exp

@@ -1104,10 +1104,16 @@ proc_with_prefix test_invalid_args {} {
     "12" \
     "TypeError.*: format_string\\(\\) takes 0 positional arguments but 1 were given.*"
 
+  # For python <= 3.12.
+  set re1 \
+    "TypeError.*: 'invalid' is an invalid keyword argument for this function"
+  # For python >= 3.13.
+  set re2 \
+    "TypeError.*: this function got an unexpected keyword argument 'invalid'"
   check_format_string \
     "a_point_t" \
     "invalid=True" \
-    "TypeError.*: 'invalid' is an invalid keyword argument for this function.*"
+    "($re1|$re2).*"
 
   check_format_string \
     "a_point_t" \