Update notes on venv installation
Updated the readme to explain that Python 3.8+ is required for Pycalibration now*, explains 2 (and a half) options: pyenv, desy spackinstance, mythical DA spack instance, if it exists.
* question: currently we still have 3.6 as the minimum version, should we bump this now or only once we start using 3.8 features?
Edited by Robert Rosca