[opt-viewer] Python 3 support in opt-diff.py

[modocache]

Summary

The file() builtin is not available in Python 3; use open() instead.
https://docs.python.org/3.0/whatsnew/3.0.html#builtins

[davide]

LGTM assuming you tested that this doesn't break python 2

[modocache]

Yup, this works with Python 2 as well. Thanks for the review, @davide!

utils/opt-viewer/opt-diff.py

if __name__ == '__main__':
 
     added = set(all_remarks2.values()) - set(all_remarks1.values())
     removed = set(all_remarks1.values()) - set(all_remarks2.values())
 
     for r in added:
         r.Added = True
     for r in removed:
         r.Added = False
-    stream = file(args.output, 'w')
+    with open(args.output, 'w') as stream:
         yaml.dump_all(added | removed, stream)