Python3 support and dropping old python support

Written for MadGraph5_aMC@NLO by Olivier Mattelaer on 2020-05-27

MadGraph5_aMC@NLO has now python3 support. Python version supported are Python2.7 and Python 3.7 (or higher).
The latest version with support of Python2.6 was MG5aMC version 2.7.3 and the beta version 3.0.3.
Support for Python2.7 is still planned up to december 2021 but some new features will only be python3 compatible before that.

Updated .

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