Remake ROOT with Python3

[Wile_E_Coyote]

Your a bit old version of ROOT supports one version of python only (specified at build time, so you need to have separate ROOT binary distributions if you want different python versions).

Newest ROOT versions have partial support for two python versions simultaneously:

Python: No module named 'root'

You MUST use EXACTLY the same Python version (and the same compiler) that your ROOT binaries were built for (otherwise, expect mysterious crashes anywhere, anytime): root-config --python-version # C/C++ calling python ; root-config --python2-version # python 2 calling C/C++ ; root-config --python3-version # python 3 calling C/C++ ; BTW. Remember to “source /the/location/of/ROOT/bin/thisroot.sh” to add your ROOT executables and libraries to your environment (when ROOT is not installed in some “…