Pythia8_i Update, master branch (2021.05.07.)
This is meant as a replacement for !43229 (closed), reducing the changeset to the bare essentials.
The update is necessary to prepare for LCG_100_ATLAS_1, which comes with pythia8 version 304. (The request for LCG_100_ATLAS_1 is discussed on https://sft.its.cern.ch/jira/browse/SPI-1881.)