make external_url_handler example py3 compliant
make external_url_handler example py3 compliant

- a raises statement with multiple values is no longer allowed in python 3

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antee/flask,pinkpet/flask,s7v7nislands/flask,elelianghh/flask,jiimaho/flask,ghhong1986/flask,jenalgit/flask,rollingstone/flask,jonatanblue/flask,kuhli/flask,nZac/flask,hoatle/flask,robertglen/flask,ghhong1986/flask,karen-wang/flask,liaoqingwei/flask,dhruvsrivastava/flask,cheesecake4392/flask,karen-wang/flask,chenke91/flask,flamelizard/flask,ZacariasBendeck/flask,nju520/flask,dhruvsrivastava/flask,xujianhai/flask,jawed123/flask,visaxin/flask,algoStruct/flask,Freebird2014/flask,Eagles2F/flask,skmezanul/flask,cgvarela/flask,pledo/flask,mAzurkovic/flask,flamelizard/flask,ryandvill/flask