Fix import for Python 2.7
Fix import for Python 2.7

shikhar413/openmc,wbinventor/openmc,amandalund/openmc,amandalund/openmc,johnnyliu27/openmc,johnnyliu27/openmc,shikhar413/openmc,mit-crpg/openmc,walshjon/openmc,liangjg/openmc,wbinventor/openmc,johnnyliu27/openmc,shikhar413/openmc,paulromano/openmc,mit-crpg/openmc,mit-crpg/openmc,smharper/openmc,walshjon/openmc,walshjon/openmc,liangjg/openmc,wbinventor/openmc,shikhar413/openmc,liangjg/openmc,mit-crpg/openmc,paulromano/openmc,paulromano/openmc,smharper/openmc,paulromano/openmc,smharper/openmc,walshjon/openmc,amandalund/openmc,johnnyliu27/openmc,smharper/openmc,wbinventor/openmc,amandalund/openmc,liangjg/openmc