[Twisted-Python] Decide on and document Python version support policy

[Tim Allen]

On 12/24/2009 02:26 PM, exarkun at twistedmatrix.com wrote:
>    * No reasonably current releases of mainstream platforms that I'm aware
> of are still bound to Python 2.3.

http://www.redhat.com/security/updates/errata/

RHEL4 is still bound to Python 2.3, and only enters the "production 3" 
phase (also known as "please, please upgrade") in 2009Q4, but is 
supported for "qualified security errata of important or critical impact 
and selected mission critical bug fixes" until February 29, 2012.

I suspect most potential and current Twisted users on RHEL4 or CentOS4 
are either using an existing version of Twisted that still works with 
Python 2.3, or already planning on upgrading to RHEL5, so "Twisted drops 
support for old versions of Python when the associated RHEL release 
enters Production 3" seems a pretty reasonable policy.

RHEL5 with Python 2.4 enters Production 3 "no earlier than Q1 of 2012". 
If we're lucky, RHEL6 will come with Python 2.6 or even 2.7 rather than 2.5.