Support current versions of python, django and djangorestframework

[SupImDos]

Summary

We need to at least be testing python 3.9 / django 4.2 / djangorestframework 3.15.1, and it seems appropriate to just be testing current (i.e., non-EOL) versions of these things.

Changes

• Updates nox config to test relevant versions of python / django / djangorestframework
• Updates development dependencies and poetry.lock file
• Fixes broken test app AppConfig (previously not actually used)

Tests

$ nox
...
nox > Ran multiple sessions:
nox > * tests-3.8(django='3.2'): success
nox > * tests-3.8(django='4.2'): success
nox > * tests-3.8(django='5.0'): skipped
nox > * tests-3.9(django='3.2'): success
nox > * tests-3.9(django='4.2'): success
nox > * tests-3.9(django='5.0'): skipped
nox > * tests-3.10(django='3.2'): success
nox > * tests-3.10(django='4.2'): success
nox > * tests-3.10(django='5.0'): success
nox > * tests-3.11(django='3.2'): success
nox > * tests-3.11(django='4.2'): success
nox > * tests-3.11(django='5.0'): success
nox > * tests-3.12(django='3.2'): success
nox > * tests-3.12(django='4.2'): success
nox > * tests-3.12(django='5.0'): success