add encoding attribute to Py2 sys.stdout

[mr-c]

Fixes #2196

[JelleZijlstra]

As @gvanrossum said in the linked issue, this would break any code trying to assign the result of an open() call to sys.stdout.

Also, stderr, stdout, and stdin are all the same type; it doesn't make sense to use a special type just for stdout.

[JelleZijlstra]

Closing since I don't think this change as it stands is an improvement.

[gvanrossum]

Agreed.