[mechanics] fix base collision/__init__ import path for Python 3
[mechanics] fix base collision/__init__ import path for Python 3

bremond/siconos,siconos/siconos,radarsat1/siconos,fperignon/siconos,bremond/siconos,fperignon/siconos,radarsat1/siconos,fperignon/siconos,fperignon/siconos,siconos/siconos,siconos/siconos,siconos/siconos,radarsat1/siconos,bremond/siconos,radarsat1/siconos,fperignon/siconos,bremond/siconos,bremond/siconos,radarsat1/siconos