Tag to be used with the 21.X branches of atlas/athena. Updates in this tag: - Backported the CMake 3.13 updates from the master branch; - Backported AtlasLCG changes necessary for LCG_95; - Updated lwtnn to version 2.8.1; - Taught AtlasCMake to remove stale .pyc files in incremental builds; - Updated AnalysisBaseExternals and AthAnalysisExternals to use LCG_95.