resolve python2 str() issue (#4030)
resolve python2 str() issue (#4030)

* make string conversion use utf8

* Update viz.py

* make utf-8 consistent with other usages

* Update viz.py

* Update viz.py

timifasubaa/incubator-superset,airbnb/superset,mistercrunch/panoramix,airbnb/superset,zhouyao1994/incubator-superset,apache/incubator-superset,timifasubaa/incubator-superset,airbnb/superset,airbnb/superset,timifasubaa/incubator-superset,dmigo/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,airbnb/caravel,mistercrunch/panoramix,airbnb/caravel,apache/incubator-superset,timifasubaa/incubator-superset,dmigo/incubator-superset,dmigo/incubator-superset,airbnb/caravel,mistercrunch/panoramix,mistercrunch/panoramix,airbnb/caravel,dmigo/incubator-superset