GitHub Actions: Re-instate coveralls.io

[lukpueh]

Description of issue or feature request:
A recent switch from Travis to GitHub Actions removed coveralls.io integration from our CI setup. (see #432)

Current behavior:
No publication of coverage data on coveralls.io
No coveralls.io badge

Expected behavior:
Re-instate publication of coverage data on coveralls.io
Re-add coveralls.io badge

** Important notes **

• See GitHub Actions: Reinstate coveralls.io integration theupdateframework/python-tuf#1243 for integration and #1287 for accompanying issues.
• Coordinate with GitHub Actions: Reinstate coveralls.io integration secure-systems-lab/securesystemslib#325 (especially re Python2 support)

[lukpueh]

This is also relevant for test branch coverage and statement coverage best practice gold badge criteria.

Let's update the corresponding entries on https://bestpractices.coreinfrastructure.org/en/projects/1523?criteria_level=2 when fixing this issue.