docs: drop Python 3.6 references
Make sure the following tools are installed and running:
- MadGraph (we have tested our setup with version 2.8.0+). See [MadGraph's website][web-madgraph-main-page]
- for installation instructions. Note that MadGraph requires a Fortran compiler as well as Python 3.6+.
+ for installation instructions. Note that MadGraph requires a Fortran compiler as well as Python 3.7+.
- For the analysis of systematic uncertainties, LHAPDF6 has to be installed with Python support
(see also [the documentation of MadGraph's systematics tool][web-madgraph-systematics]).
