Python 3 - Use universal_newlines=True in subprocess.Popen.

This follows the spirit of a previous patch which did essentially
the same thing.  In Python 3, when you use Popen.communicate(),
you get back a bytes object which cannot normally be treated as
a string.  We could decode this manually, but universal_newlines=True
does this automatically, and there's no disadvantage to doing so
even on Python 2.  So just enable it always.

llvm-svn: 252126