Update py3 and new cmake slots

As discussed at Core Software Meeting, the lhcb-gaudi-head-py3 slots builds for Python 2 too.

I also updated the configuration so that both lhcb-gaudi-head-py3 and lhcb-new-cmake inherit the lhcb-gaudi-head merge requests (on top of their own), and I also build for Python 3 in lhcb-new-cmake.