util: Remove Python 2.7 glue code

Remove uses of six and from __future__ imports as they are no longer
needed.

Change-Id: I8d6aae84d8192b301d541b8dc81275f4932f9f2f
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39756
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/style/verifiers.py

@@ -47,8 +47,6 @@ import os
 import re
 import sys
 
-from six import add_metaclass
-
 from . import style
 from . import sort_includes
 from .region import *
@@ -102,8 +100,7 @@ def _modified_regions(old, new):
     return regions
 
 
-@add_metaclass(ABCMeta)
-class Verifier(object):
+class Verifier(object, metaclass=ABCMeta):
     """Base class for style verifiers
 
     Verifiers check for style violations and optionally fix such
@@ -224,7 +221,6 @@ class Verifier(object):
         """
         pass
 
-@add_metaclass(ABCMeta)
 class LineVerifier(Verifier):
     def check(self, filename, regions=all_regions, fobj=None, silent=False):
         close = False