Publish a last `cose` release

[letmaik]

The text on https://pypi.org/project/cose/ is confusing as it says that pycose is old code. Since this has been fixed now and pycose is the way forward, there should be a last release for cose to update the README on PyPI to say that this package won't receive further updates and people should switch to pycose.

I think this means:

• Create a branch off of v0.9.dev8, maybe named release/0.9.x
• Create a PR branch targeting release/0.9.x
• Make the necessary README changes in that branch
• Edit the version in setup.py to 0.9.dev9
• Create a PR for review & merge it into release/0.9.x
• Create a GitHub release with git tag v0.9.dev9 off of release/0.9.x
• Push to PyPI

[letmaik]

@TimothyClaeys Are you happy for me to create the target branch and PR? Is there anything I haven't considered above?