Add explicit six dependency (#4698)
Add explicit six dependency (#4698)

Closes #4695
azavea/raster-foundry,raster-foundry/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,aaronxsu/raster-foundry,raster-foundry/raster-foundry,azavea/raster-foundry,raster-foundry/raster-foundry,aaronxsu/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,azavea/raster-foundry