Cogeo-mosaic

* no change since `4.0.0a2`

* update rio-tiler requirement (`>=3.0.0a6`) and update backend reader type information

* update rio-tiler requirement (`>=3.0.0a5`)
* fix `MosaicBackend` to match Backend input names.

* update morecantile requirement to >= 3.0
* update rio-tiler requirement to >= 3.0 and update Backend's properties
* switch from `requests` to `httpx`
* add `BaseBackend.assets_for_bbox()` method (https://github.com/developmentseed/cogeo-mosaic/pull/184)

**breaking changes**

* remove `BaseBackend.metadata()` method (can be replaced by `BaseBackend.mosaic_def.dict(exclude={"tiles"})`)
* remove `cogeo_mosaic.models.Metadata` model
* remove python 3.6 support
* `BaseBackend.path` -> `BaseBackend.input` attribute (`input` was added in rio-tiler BaseReader)

* Add Google Cloud Storage (`gs://...`) mosaic backend (author AndreaGiardini, https://github.com/developmentseed/cogeo-mosaic/pull/179)

* Make sure to pass an openned file to click.Progressbar (https://github.com/developmentseed/cogeo-mosaic/pull/178)