kunit: add fallback for os.sched_getaffinity

Python 3.13 added os.process_cpu_count as a cross-platform alternative
for the Linux-only os.sched_getaffinity. Use it when it's available and
provide a fallback when it's not.

This allows kunit to run on macOS.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>