ion-python documentation is out of date on readthedocs.org

[popematt]

See https://readthedocs.org/projects/ion-python/builds/

The latest successful build of the docs is from April 2021.

[cheqianh]

I haven't looked too closely at the document release process, one possible reason is that we dropped the support for python 2 last year but it's still using python2 CLI to release document.