Move over to using poetry for packaging and pre-commit for code checking

Closes #3

See merge request !5

This commit changes no functionality but add pre-commit checks based on
our usual template and fixes up the code to match. Note that this found
a real bug in the type annotation for the override_permissions_spec
decorator.

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

Support later Django versions

Closes #1

See merge request !3

Update (or, more correctly, create) the changelog in the process.

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.

Downgrade required version of ucam-identitylib and be more permissive about required versions

See merge request !2

This is not ideal, but given that we only need the identifiers from identitylib
the downgrade in version and more permission version allows us to use this
library with libraries which are making use of different versions of identitylib
without needing to upgrade identitylib and refactor.

Initial implementation

See merge request !1