Supported python versions, pre-commit tests, sphinx

[Andrew Bennetts]

Robert Collins wrote:
> Currently we support python2.4 and up, and build optional docs via
> sphinx. sphinx seems to be nicer than what we were using before, and
> Ian wants to delete the old cruft:)
> 
> However our pre-commit testing environment currently runs 'hardy', and
> that does not have python-sphinx available.
> 
> We have a few of [easyish] options.
>  - don't make this change
>  - stop testing that the docs build
>  - backport sphinx to hardy
>  - change the test environment to run 'lucid'

Of these, “backport sphinx to hardy” seems best to me.  The drawbacks of
the other options are too great.

-Andrew.