1.6.0 release

[sgillies]

New features like an option to keep empty tiles and create an alpha band for RGB input which has a mask of some kind.

[sgillies]

1.6a1 release going to PyPI today.

[sgillies]

https://pypi.org/project/rio-mbtiles/1.6a1/#files.

[sgillies]

Here's 1.6a2 https://pypi.org/project/rio-mbtiles/1.6a2/#files

[sgillies]

Target date for 1.6.0 is Wednesday, 2021-07-28.