python: Make exception handling Python 3 safe

Change-Id: I9c2cdfad20deb1ddfa224320cf93f2105d126652
Reviewed-on: https://gem5-review.googlesource.com/c/15980
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/python/m5/util/code_formatter.py

@@ -74,7 +74,7 @@ class lookup(object):
             return self.args[item]
         except ValueError:
             pass
-        raise IndexError, "Could not find '%s'" % item
+        raise IndexError("Could not find '%s'" % item)
 
 class code_formatter_meta(type):
     pattern = r"""