CDRIVER-4641: A Poetry project file (#1276)

* A basic Poetry project for managing dev scripts and tools
  Fixup several other scripts to be set -u safe since we now import that
  setting.
* Use Poetry to run Sphinx
* Change variant used for release archive
  The release archive build invokes Sphinx, and since Sphinx is now installed by
  Poetry, we require a newer Python version. make-release-archive now runs
  on Ubuntu 22.04
* abi-compliance-check needs u18
  The abi-compliance-check task assumes an external `abi-compliance-checker`
  tool is installed, but that is only true on the Ubuntu 18.04 EVG
  distros.
* Significant script refactoring:
- Split components into separate files, and provide a way to selectively
  import them from other scripts.
- Download install-poetry.py on-the-fly when it is needed, not committed
  into the repository.
- Add a download-file utility.
- python.sh/poetry.sh are importable to be reused in other scripts.
* Add notes about building docs to CONTRIBUTING
* Parallelize the HTML build
* Add a console script for EVG generation