Update Build-Depends and add pyverions

- Update Build-Depends to require OpenAFS 1.6 or higher (this will
  not build on 1.4 and we don't want to try).
- Add X-Debathena-Nobuild for squeeze and lucid which are too old
- Add debian/pyversions to enforce the 2.6 requirement added in
  7b37633d
- Update the changelog to clarify we no longer support old OpenAFS