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/m5/util/code_formatter.py

@@ -154,7 +154,7 @@ class code_formatter(object):
         self._data = []
 
     def write(self, *args):
-        f = file(os.path.join(*args), "w")
+        f = open(os.path.join(*args), "w")
         for data in self._data:
             f.write(data)
         f.close()