python: Switch from using compare to key in list sort

python: Switch from using compare to key in list sort

Python 3 has deprecated the use of a comparison function in favour of
a key extraction function.

Change-Id: I4b7eab791ecbdfbf7147f57fdbc7cbe8f1de20dd
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15995
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>