python: Add AbstractStat for PyStats

Previously all PyStats inheritted from JsonSerializable. The
AbstractStat class has been added to give a cleaner, clearer Base class
for PyStats.

Change-Id: I7e1808c4b4dcd6110fd524ad3553a9dc19f72e24
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63691
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
6 files changed