[Urwid] Dropping support for Python 2.1

[Ian Ward]

I am planning to drop support for Python 2.1 in a release of Urwid 
between now and 1.0.0.

When I first released Urwid, Python 2.1 was the standard version 
installed with Debian and OS X, but now it is much less common.  I am 
planning a feature that will make use of metaclasses, and I would like 
to purge some of the old-version-cruft that has been accumulating.

If this change will affect you please let me know and I will try to 
write the new feature in a way that doesn't break under python 2.1 
(though the feature itself won't work), and I will put off dropping 
support for Python 2.1 until after 1.0.0.

Ian