Python 3.5 is scheduled to reach its end of life in September. See https://devguide.python.org/#status-of-python-branches.

To stay ahead of the curve, I think we should deprecate support for Python 3.5 now and then remove support for it in our release in the beginning of September.

You can find an example of how we've done this in the past at #7378. In addition to what was done there though, since there is some support for using Certbot as a library, I think we should add a warning to Certbot's __init__.py like we do in acme too.