Fix #470 'imp' module PendingDeprecationWarning
Fix #470 'imp' module PendingDeprecationWarning

The package 'imp' will be deprecated in favor of importlib. This change allows to use os 'importlib' when python version is greater than 2 or use 'imp' otherwise.
ponyorm/pony,ponyorm/pony,ponyorm/pony,ponyorm/pony