Cedar-backup3

* Fix obsolete GitHub URL in several places.

* Minor adjustments to PyPI package documentation.
* Execute 'sudo -v' prior to running full test suite.
* Fix release process to work consistently across platforms.

* Convert to modern Python packaging based on Poetry.
* Apply code formatting standards with Black and isort.
* Switch to standard unittest discovery process to run test suite.
* Convert user manual from Docbook XML to Sphinx and revise content.
* Migrate all documentation (manual and API specs) to Read the Docs.
* Add a GitHub workflow to implement code quality gates, executed via Tox.
* Clean up and remove obsolete and unused notes, utilities, etc.
* Fix resource leak in executeCommand() that was exposed by unit tests.
* Increase minimum interpreter version from Python 3.4 to Python 3.7.

* Migrate to GitHub now that BitBucket no longer supports Mercurial repositories.
* Handle new warnings from PyLint 2.3.1.

* Fix MANIFEST.in so that all API documentation is included in tarball.

* Convert API documentation from Epydoc to Sphinx Napolean (Google docstrings).