Tiers manifest files are to help planing packages update in advance.

- 1-3 :: Packages with high impact ratio and mainly updated on
         the python-team branch.
- 4 :: Packages with high number of inputs, building which ensure the
       health of large portion of the graph.
- 5 :: All leaf packages which safe to update directly on master branch.

They may be combined to query for substitute availability during work on
the branch:

    for t in $(seq 5)
    do
        guix time-machine --branch=python-team -- \
            weather --display-missing \
            --substitute-urls=https://ci.guix.gnu.org \
            --manifest="etc/teams/python/python-tier-${t}-manifest.scm"
    done

* etc/teams/python/python-tier-1-manifest.scm: New file.
* etc/teams/python/python-tier-2-manifest.scm: New file.
* etc/teams/python/python-tier-3-manifest.scm: New file.
* etc/teams/python/python-tier-4-manifest.scm: New file.
* etc/teams/python/python-tier-5-manifest.scm: New file.

Fixes: guix/guix#9584
Merges: guix/guix!9601
Reviewed-by: Nicolas Graves <ngraves@ngraves.fr>