python: Switch to using open instead of file

Python 3 doesn't support the file(name, mode) syntax which has been
deprecated in favour of open.

Change-Id: I35ef8690d97a5243860a64ff985fd22fa86253f1
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15985
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/python/importer.py

@@ -67,7 +67,7 @@ class CodeImporter(object):
 
             override = os.environ.get('M5_OVERRIDE_PY_SOURCE', 'false').lower()
             if override in ('true', 'yes') and  os.path.exists(abspath):
-                src = file(abspath, 'r').read()
+                src = open(abspath, 'r').read()
                 code = compile(src, abspath, 'exec')
 
             if os.path.basename(srcfile) == '__init__.py':