Tag to be used with the master branch of atlas/athena. Update(s) since 2.0.26: - Made CheckerGccPlugins only work with GCC 8, and implemented a new fix for one of its checks; - Updated to lwtnn 2.8.1; - Updated to Acts 0.8.2; - Added prmon to AthSimulationExternals and AthDerivationExternals; - Taught the CMake configuration step clean out any stale .pyc files from the build directory, whose .py files have been removed from the source.