Cedar-backup3

* Move to newer build dependencies (Black, isort, etc.).
* Make adjustments to code formatting to match new Black version.
* Force docs/requirements.txt to have UNIX line endings
* Convert to PEP 440 convention for Git tags (i.e. "v3.5.3").
* Standardize and improve badges in markdown, so they are all links.
* Simplify pylint configuration and use "-j 0" to speed it up.

* Improve debug output for pre/post action hooks.

* Adjust .gitattributes to force UNIX line endings, for compatibility.

* Changes to support the Windows platform, for the first time since 3.0.1.
* Minor tweaks to the codebase to make the PyCharm code inspector happy.

* Fix Python dependencies to work with Python 3.9.1 and not just 3.9.0.

* Add a new -u/--uploadOnly option to cback3-amazons3-sync.