Implement Last Serial for Python Repositories

[gerrod3]

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.