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/importer.py

@@ -35,7 +35,7 @@ class CodeImporter(object):
 
     def add_module(self, filename, abspath, modpath, code):
         if modpath in self.modules:
-            raise AttributeError, "%s already found in importer" % modpath
+            raise AttributeError("%s already found in importer" % modpath)
 
         self.modules[modpath] = (filename, abspath, code)