macOS Big Sur Updates, master branch (2020.11.15.)

These are all the changes that I needed to make to get AnalysisBaseExternals to build on the newly released macOS Big Sur.

Most importantly I had to switch from Python 3.7.6 to 3.8.6, and I had to backport some fixes from the ROOT git repository on top of 6.22/02. Everything else was either a purely technical update, or a consequence of the Python update.

Pinging @krumnack and @emoyse, who may be interested in this. 😉 Note that of course I only tested this on x86. Since I won't be getting an ARM -- excuse me, Apple Silicon -- Mac just yet, any updates necessary for those will have to come from someone else...