Chores: testing, versioning, publishing

[thekaveman]

• Use coverage directly, and add coverage-comment to workflow
• Dynamic version from git metadata (tags)
• Publish to PyPI and release to GitHub on tag
• Publish to PyPI using a Trusted Publisher
• Devcontainer cleanups, workflow cleanups

❗ Breaking Changes ❗

• Drop support for Python 3.7 -- minimum version 3.8 (to enable dynamic versioning)

• Following up to feat: Validate multi-line messages #68, a blank line between the commit subject and body is now required per the Spec

  Prior to this change, the following commit would pass (unless the --strict arg was used):

  $ git commit -m "feat: subject line
  body line with no blank in the middle"

  After this change, the --strict arg has no effect, commits with bodies must be formatted with a blank line:

  $ git commit -m "feat: subject line
  
  body line with blank in the middle"

Because of these changes, the next version release will be v3.0.0