Maintenance release v2.3 Feature fixed on v2.3: * fix compatibility with dnspython 2.0 (#11) * fix issue when trying to apply DNS update on secondary server (thanks Sebastian Koechlin) * fix debug message when CNAME was not found * fix test chain assertion (a chain contains more than 1 certificate) Feature added: * skip already validated authorization challenge Continuous Integration improvement: * Debian Stretch tests were using Debian Jessie image * Enable Docker build uses [BUILDKIT](https://docs.docker.com/develop/develop-images/build_enhancements/) and the `overlay2` filesystem driver Other: * the .gitignore file has been simplified