fix: use io.StringIO for python3 compatibility
fix: use io.StringIO for python3 compatibility

oscarlorentzon/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM,oscarlorentzon/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM