Updates for PyROOT and RDataFrame changes in v6.22

These are quite localised and compatible with the v6.20/02 we get through LCG_97python3, but I wouldn't merge this until 6.22 is released - mostly pushing in case anyone else wants to test with a more recent version (a daily build of the master branch is available in dev3python3; it does require /cvmfs/sft-nightlies.cern.ch).

Update: not officially announced yet, but the release notes for LCG_98python3 are available here