update PowhegControl interface to the version from rel. 21.6, some fixes to Herwig7 common fragments

PowhegControl interface was improved some time ago and we run the new version in 21.6 MC in production This new version is now moved to master + some fixes required by Python3 are added

some fixes to Herwig7 common fragments - required by Python3 - are added

This MR is needed for https://its.cern.ch/jira/browse/AGENE-1963 and https://its.cern.ch/jira/browse/AGENE-2012