fail.py: handle missing cgitb

It's no longer in python standard library starting
at version 3.13. Fixes:

root@qemuarm64:~# aa-complain /etc/apparmor.d/*
Traceback (most recent call last):
  File "/usr/sbin/aa-complain", line 18, in <module>
    from apparmor.fail import enable_aa_exception_handler
  File "/usr/lib/python3.13/site-packages/apparmor/fail.py", line 12, in <module>
    import cgitb
ModuleNotFoundError: No module named 'cgitb'

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>