lib.graph: revert to backported WeakSet if not available (python 2.6)
lib.graph: revert to backported WeakSet if not available (python 2.6)

carlye566/IoT-POX,PrincetonUniversity/pox,jacobq/csci5221-viro-project,kavitshah8/SDNDeveloper,jacobq/csci5221-viro-project,chenyuntc/pox,diogommartins/pox,kulawczukmarcin/mypox,adusia/pox,xAKLx/pox,andiwundsam/_of_normalize,adusia/pox,chenyuntc/pox,carlye566/IoT-POX,denovogroup/pox,adusia/pox,denovogroup/pox,VamsikrishnaNallabothu/pox,pthien92/sdn,MurphyMc/pox,VamsikrishnaNallabothu/pox,MurphyMc/pox,kavitshah8/SDNDeveloper,PrincetonUniversity/pox,PrincetonUniversity/pox,kpengboy/pox-exercise,kulawczukmarcin/mypox,kpengboy/pox-exercise,waltznetworks/pox,pthien92/sdn,andiwundsam/_of_normalize,diogommartins/pox,kavitshah8/SDNDeveloper,VamsikrishnaNallabothu/pox,diogommartins/pox,adusia/pox,kavitshah8/SDNDeveloper,denovogroup/pox,noxrepo/pox,denovogroup/pox,kpengboy/pox-exercise,chenyuntc/pox,chenyuntc/pox,MurphyMc/pox,xAKLx/pox,jacobq/csci5221-viro-project,noxrepo/pox,adusia/pox,andiwundsam/_of_normalize,diogommartins/pox,xAKLx/pox,jacobq/csci5221-viro-project,carlye566/IoT-POX,VamsikrishnaNallabothu/pox,VamsikrishnaNallabothu/pox,kulawczukmarcin/mypox,PrincetonUniversity/pox,pthien92/sdn,xAKLx/pox,noxrepo/pox,chenyuntc/pox,diogommartins/pox,MurphyMc/pox,waltznetworks/pox,waltznetworks/pox,pthien92/sdn,kpengboy/pox-exercise,jacobq/csci5221-viro-project,carlye566/IoT-POX,xAKLx/pox,carlye566/IoT-POX,waltznetworks/pox,PrincetonUniversity/pox,denovogroup/pox,kulawczukmarcin/mypox,MurphyMc/pox,kulawczukmarcin/mypox,kpengboy/pox-exercise,noxrepo/pox,pthien92/sdn,andiwundsam/_of_normalize,waltznetworks/pox