2.5.6

This release finally builds again on Windows. Binary packages for major Linux distributions, an AppImage version, a Windows installer, a Homebrew package and a Docker image are all built automatically on github actions from now on, and will be available as release assets shortly after each release. There are no new features in this version, but the code to locate the Python installation has been cleaned up substantially, which may help other package builders (no more reference to hard-coded paths in the cadabra2 binaries).