Port packaging to poetry. This does not make any changes to the existing
test matrix or requirements.

We now have common Python project CI templates which, while continuing
to support PyPI publication, also allow for parallel matrix testing with
tox.

The GitLab CI configuration in this repo dates from a time before this
feature was in GitLab (as noted in the comments) and so had to work
around it by creating a lot of templated jobs.

Simplify the CI configuration by making use of the common template.