Remove support for older Python versions.

[Lukasa]

This removes the block talking about supporting Pythons older than 2.2 (because let's be honest, we're all well past that now).

It replaces it with a block that aims to be somewhat forward looking, restricting the versions of Python that WSGI 1.1 supports.

[andrewgodwin]

I think this is sensible - part of me wonders if tying it to CPython is a bit short-sighted, but then I don't see it going away.

[cdent]

👍

[benoitc]

👍