Tolerate python 2.6
Tolerate python 2.6

importlib doesn't exist in python2.6. We can tolerate this missing if we don't support hoomd_external_plugins

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue