Fix test_tablesMD.py to work with python 3
Fix test_tablesMD.py to work with python 3

rabernat/PyTables,rdhyee/PyTables,mohamed-ali/PyTables,dotsdl/PyTables,jennolsen84/PyTables,rabernat/PyTables,jennolsen84/PyTables,andreabedini/PyTables,avalentino/PyTables,joonro/PyTables,jack-pappas/PyTables,mohamed-ali/PyTables,jack-pappas/PyTables,joonro/PyTables,cpcloud/PyTables,FrancescAlted/PyTables,joonro/PyTables,dotsdl/PyTables,cpcloud/PyTables,PyTables/PyTables,PyTables/PyTables,andreabedini/PyTables,gdementen/PyTables,gdementen/PyTables,jack-pappas/PyTables,jack-pappas/PyTables,jennolsen84/PyTables,rabernat/PyTables,rdhyee/PyTables,avalentino/PyTables,mohamed-ali/PyTables,tp199911/PyTables,rabernat/PyTables,FrancescAlted/PyTables,dotsdl/PyTables,cpcloud/PyTables,FrancescAlted/PyTables,jack-pappas/PyTables,cpcloud/PyTables,andreabedini/PyTables,tp199911/PyTables,tp199911/PyTables,joonro/PyTables,gdementen/PyTables,andreabedini/PyTables,mohamed-ali/PyTables,jennolsen84/PyTables,rdhyee/PyTables,PyTables/PyTables,gdementen/PyTables,dotsdl/PyTables,rdhyee/PyTables,tp199911/PyTables,avalentino/PyTables