GitHub Actions: Reinstate coveralls.io integration

[lukpueh]

Description of issue or feature request:
Current behavior:
A recent switch from Travis to GitHub Actions removed coveralls.io integration from our CI setup. (see #324)

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

Expected behavior:
Re-instate publication of coverage data on coveralls.io (see theupdateframework/python-tuf#1252)
Re-add coveralls.io badge (revert 2fac344)

NOTE: I suggest to hold off until we drop Python 2 support. It will make our lives easier. (see theupdateframework/python-tuf@8bb0187)