Fix Python 3 build

!646 (merged) introduced a regression in the Python 3 build

<unkown>: error: AttributeError: 'dict_values' object has no attribute 'to_json'
PyConf/python/PyConf/components.py:70: error: TypeError: dict_values(['/Event/HLT2/Rec/ProtoP/Charged', '/Event/HLT2/Rec/ProtoP/Neutrals', '/Event/HLT2/Rec/Track/Best', '/Event/HLT2/Rec/Vertex/Primary', '/Event/HLT2/Rec/Calo/Electrons', '/Event/HLT2/Rec/Calo/Photons', '/Event/HLT2/Rec/Calo/MergedPi0s', '/Event/HLT2/Rec/Calo/SplitPhotons', '/Event/HLT2/Rec/Muon/MuonPID', '/Event/HLT2/Rec/Rich/PIDs']) is not json serializable

The only 'bare', non-iterated dict.values() call I can see in that MR is here, so hopefully just wrapping that with a list() call will fix the problem.