util: Port git hooks to python3

util: Port git hooks to python3

This involves changing:
* git-commit
* git-pre-commit
* style verifiers

JIRA: https://gem5.atlassian.net/browse/GEM5-473

Change-Id: I7bd0b54469f942bf927c8be1fd94d12f67594d48
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28588
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>