[gdb/testsuite] Fix gdb.python/py-mi-cmd.exp

On Fedora Rawhide aarch64-linux (using Python 3.15.0b4), with test-case
gdb.python/py-mi-cmd.exp I ran into:
...
Expecting: ^(-pycmd bk3[^M
]+)?(&"TypeError.*: __repr__ returned non-string \(type BadKey\).."^M
\^error,msg="Error occurred in Python: __repr__ returned non-string \(type BadKey\)"[^M
]+[(]gdb[)] ^M
[ ]*)
-pycmd bk3^M
&"TypeError: ReallyBadKey.__repr__() must return a str, not BadKey\n"^M
^error,msg="Error occurred in Python: ReallyBadKey.__repr__() must return a str, not BadKey"^M
(gdb) ^M
FAIL: $exp: -pycmd bk3 (unexpected output)
...

Fix this by updating the regexp.

Tested with aarch64-linux (Python 3.15.0b4) and x86_64-linux (Python 3.13.14).

Approved-By: Tom Tromey <tom@tromey.com>