Release v0.1.15

Changes

• 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