Implement Last Serial for Python Repositories

Original Pulp Redmine Issue: https://pulp.plan.io/issues/7491

When syncing with PyPi a check for the field last_serial is performed to see if an update is needed. Currently this field is defaulted to 0 in the sync operation so the update is always performed, but this optimization can be added by creating a last_serial field for each Python repository. This field would be passed onto the sync operation and would update after each sync. This field is currently hard-coded to 1000000000 in the PyPi live API when Pulp distributes Python content, but this value can be switch to the repository's value after it is created.

Start with the Python repository sync operation and the PyPi live API behavior described in the issue. Trace how last_serial is currently defaulted to 0 during sync and hard-coded to 1000000000 in the live API. Done means each repository stores and passes its value through sync, updates it after syncing, and the live API uses the repository value.