Python CI Upgrades for GitLab 15.0

These changes incorporate new updates to the CI logic to work with GitLab version 15.0. This MR won't work with the existing version, 14.10, and shouldn't be merged until the upgrade -- scheduled for May 28, 2022.

Coverage Report

The old report type, cobertura, is no longer recognized. Instead, use the newer report type coverage_report with the format set to cobertura. Behavior is the same.

Python Image

Gemnasium Python is now on Python 3.9 by default. There's no need for the special override -- that image was an emergency stop-gap GitLab released to enable 3.9 development in a non-breaking way.

Testing

Here's a Multi Pipeline demonstrating these changes with the GitLab scanners. The one failing project is still very young, and is also failing it's main branch.