Remove py3 slots and add gcc9+py3-opt to master
As agreed in PAC, we remove the lhcb-head-py3 and lhcb-run2-patches-py3 as the stack is largely supporting Python 3 by now.
In addition, lhcb-master (and thus ci-test) gets one py3 platform so that we make sure Python 3 support is not broken.