Add python 3 build to lhcb-head, lhcb-gaudi-head, lhcb-run2-patches slots

Now that the LHCb stack is mostly Python 3 compliant, add python3 build to the regular testing slots, to ensure we avoid regressions.

The dedicated (temporary) Python 3 slots are retained while working to complete the porting to Python 3

@clemenci @dovombru FYI