PEP 3131: Supporting Non-ASCII Identifiers

["Martin v. Löwis"]

>    PEP 3131 uses a similar definition to C# except that PEP 3131
> disallows formatting characters (category Cf). See section 9.4.2 of
> http://www.ecma-international.org/publications/standards/Ecma-334.htm

UAX#31 discusses formatting characters in 2.2, and recognizes that
there might be good reasons to allow (and ignore) them; however,
it recommends against doing so except in special cases.

So I decided to disallow them.

Regards,
Martin