Going to throw another hat into the ring here: I use acme-tiny [1], which is a single file ACME client written in Python in under 200 lines. The idea behind it is that you can fully read and understand everything it does without spending too much time on it. I really like this approach, so I went ahead and started using it, and have been for a few years now.
[1] https://github.com/diafygi/acme-tiny