release tasks

[petrelharp]

For next time:

• - make sure you're on main
• - increment the version in pyslim/_version.py
• - update CHANGELOG
• - commit and tag
• - update tags on github with git push upstream; git push --tags upstream
• make a release on github
• - upload to pypi:

python3 setup.py sdist bdist_wheel
twine upload dist/FILES_TO_UPLOAD

• - check on pypi that the update actually worked
• - check tskit-site build succeeded
• - upload to zenodo
• - update version and DOI in CITATION.cff (once added)
• - conda-forge: will do an automatic PR (winni2k says:
  "The key things I checked were that the version number is correct, the SHA sum has changed, and that the build number is reset to zero. Any other problems should be caught by the automated build system.") or if not, make a PR that updates recipe/meta.yml with the version number and the SHA of the download from pypi
• - email SLiM list
• - check conda-forge worked: https://anaconda.org/conda-forge/pyslim
• update code in tskit-site workflow to get right pyslim/SLiM version if necessary
• make copy of this issue for the next release
• add 'upcoming' to CHANGELOG

Reference: https://packaging.python.org/tutorials/packaging-projects/