We have a first alpha pre-release on PyPI now. Many of the changes are noted at https://github.com/mapbox/rasterio/blob/master/CHANGES.txt#L7. It's not quite complete, so please see https://github.com/mapbox/rasterio/milestone/103 for what new features are or are not yet in this release. Please remember that some things may be removed or altered a bit before 1.3.0. I'm eager to hear whether rasterio's merge tool is working better in 1.3a1.
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