Improve visibility/utility of coverage report

[bhrutledge]

As of #943, Twine is no longer using CodeCov to report on coverage. The per-file coverage report is visible in the workflow run log, but it'd be nice if it were more visible, and more granular. The comments of #658 have some thoughts, but nothing really useful.

The comments at #943 (comment) suggest using GHA job summaries using Markdown output from coverage.py (which will be native in v7).

Hynek Schlawack wrote an in-depth article, which includes uploading an artifact for the excellent coverage HTML report, which supports contexts for identifying what source code executed a line of test code.