Minimum required Python version in JHBuild

[James Henstridge <james jamesh id au>]

At the moment, jhbuild requires Python >= 2.0.

I was wondering if it would inconvenience anyone if I increased the
minimum version requirement to 2.2 or 2.3 at some point.

For reference, 2.2 was released on 21 December, 2001 (3.5 years ago),
and 2.3 was released 29 July 2003 (2 years ago).

James.