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/main.py

@@ -407,7 +407,7 @@ def main(*args):
     sys.path = [ os.path.dirname(sys.argv[0]) ] + sys.path
 
     filename = sys.argv[0]
-    filedata = file(filename, 'r').read()
+    filedata = open(filename, 'r').read()
     filecode = compile(filedata, filename, 'exec')
     scope = { '__file__' : filename,
               '__name__' : '__m5_main__' }