misc: string.join has been removed in python3

In general string methods are deprecated in favour of str ones

Change-Id: Ifba04e0b70be29e5a82a67cf11837f740de57e32
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26244
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/util/code_formatter.py

@@ -35,7 +35,6 @@ except ImportError:
 import inspect
 import os
 import re
-import string
 
 class lookup(object):
     def __init__(self, formatter, frame, *args, **kwargs):
@@ -164,7 +163,7 @@ class code_formatter(object):
         f.close()
 
     def __str__(self):
-        data = string.join(self._data, '')
+        data = ''.join(self._data)
         self._data = [ data ]
         return data