Fix Python 3-related error in test_trace
Fix Python 3-related error in test_trace

bhermanmit/openmc,shikhar413/openmc,amandalund/openmc,mit-crpg/openmc,paulromano/openmc,liangjg/openmc,amandalund/openmc,mjlong/openmc,lilulu/openmc,samuelshaner/openmc,kellyrowland/openmc,wbinventor/openmc,kellyrowland/openmc,johnnyliu27/openmc,mjlong/openmc,johnnyliu27/openmc,amandalund/openmc,mit-crpg/openmc,wbinventor/openmc,smharper/openmc,shikhar413/openmc,walshjon/openmc,wbinventor/openmc,paulromano/openmc,liangjg/openmc,samuelshaner/openmc,bhermanmit/openmc,liangjg/openmc,mit-crpg/openmc,liangjg/openmc,lilulu/openmc,walshjon/openmc,samuelshaner/openmc,johnnyliu27/openmc,amandalund/openmc,walshjon/openmc,samuelshaner/openmc,paulromano/openmc,mit-crpg/openmc,wbinventor/openmc,shikhar413/openmc,paulromano/openmc,smharper/openmc,smharper/openmc,johnnyliu27/openmc,smharper/openmc,lilulu/openmc,shikhar413/openmc,walshjon/openmc