#97 Make python3 invokactions more explicit and use more Django 5 conventions

* Upgrade to the latest Django 5.1.3 * The standard way to invoke Python is `python3` explicitly. macOS warns about 2 vs 3 ambiguity when just calling `python`. * Added a `make ready` check that calls `python3 manage.py check` to catch common settings and ImproperlyConfigured errors. * Several improvements to the admin.py that uses the newer Django 5 conventions * Added a migration for changing the default PK to BigAutoField

lgtm