Apologiesserver

* Publish coverage data to coveralls.io in GitHub workflow.
* Fix incorrect dependency on safety (should have been dev dependency).

* Upgrade to cattrs 1.2.0 and handle related behavior changes.
* Update build process to include security scans using Safety.
* Make other structural improvements to the build process.

* Update build system to use poetry.core.
* Fix the GitHub test suite by adding Python version into cache id.
* Move to newer versions of various build dependencies (black, isort, etc.).
* Standardize on UNIX line endings in Git working copy and PyPI package.
* Make improvements to Read the Docs integration, including a PR merge gate.
* Clean up badges shown in documentation, so they are links to something sane.
* Make improvements to the MyPy and Pylint configuration.
* Move to a new two-step release process based on cedar-backup3.
* Create a changelog for the first time, along with new release process.
* Upgrade to latest apologies library (v0.1.29) to stay up-to-date.