[Python-3000] Support for PEP 3131

["Martin v. Löwis"]

> Sincere question: if these characters aren't needed, why are they
> provided? From what I can tell by googling, they're needed when, e.g.,
> Arabic is embedded in an otherwise left-to-right script. Do I have
> that right?

I think not. In principle, each character has a directionality
(available through unicodedata.bidirectional), and a rendering
algorithm should be able detect runs of characters that differ
in directionality from the surrounding text, rendering it
properly. As a special case, certain characters are declared
"neutral", extending the run across, say, spaces.

So embedding Arabic in an LTR text *alone* makes no requirement for
these control characters. I'm unsure whether there are cases where
the standard BIDI algorithm would produce incorrect results;
it's certainly the case that not all tools implement it correctly,
so the control characters can help those tools (assuming the
tool implements the control character at least).

Regards,
Martin