[Python-3000] More uniform treatment of files' newlines attribute?

[skip at pobox.com]

While editing the documentation of the builtin open function, I noticed that
the newlines attributes can take on three different value types: None,
strings or tuples of strings.  It seems to me it would be better if was
always a set containing the newline values seen so far.  There's no testing
necessary if you need to do something with the newlines you've seen, you
just loop over them:

    for nl in f.newlines:
        print("%r" % nl)

With the current mixed types metaphor you have to do something like this:

    if f.newlines is not None:
        if type(f.newlines) is tuple:
            for nl in f.newlines:
                print("%r" % nl)
        else:
            print("%r" % f.newlines)

This, of course, assumes the file has been opened in text mode.  If you have
a binary mode file you also have to call hasattr(f, "newlines").  Presumably
in most cases you'll know the file's mode without needing to check, but
maybe binary files should also have a newlines attribute which is always the
empty set.

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