Use Rasterio to create webp

in https://github.com/mapbox/rio-rgbify/blob/31bd65b175938a4362e0e8bf1d3b86d5e8d31e5b/rio_rgbify/mbtiler.py#L46-L69 we use Pillow to save the image as webp. Since rasterio ~1.0.9 , rasterio wheels are shipped with Webp driver so we could use rasterio instead of PIL and save on the dependency ;-)

cc @dnomadb

Start with rio_rgbify/mbtiler.py at lines 46-69, where Pillow currently saves the image as WebP, and check how rasterio's WebP driver can perform the same operation. Done means WebP output still works through rasterio and Pillow is no longer needed as a dependency.

Written by the indexing model from the issue text.