Supported python versions, pre-commit tests, sphinx

[Robert Collins]

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'

These have different tradeoffs.
If we stop testing that docs build, we pretty much guarantee that
errors will creep in.

If we backport sphinx to hardy, we'll be able to commit test things,
but we won't be able to *build bzr* on a normal hardy system (and
hardy is still important to us, I think - its supported until 2011 on
the desktop). But perhaps that doesn't matter, or perhaps we should
start building docs as part of shipping a tarball, to reduce the
dependencies end users need.

If we change the test environment to run 'Lucid' we would no longer be
checking python2.4 compatibility on every mainline commit - and I
think that this is actually a pretty large risk.

Anyhow, I don't have a particular course to recommend, and I think we
need some discussion about this, which is why I've pulled it here from
the merge proposal that Ian had put forward.

Cheers,
Rob