> I personally see code on the backend as the source of truth
That is a perfectly valid approach, although when you need to coordinate development between multiple teams starting with a specification can be really helpful. That doesn't mean that the spec has to be written once and frozen, as long as everyone is aware of the changes and adapts accordingly.
> Might be worth putting a comparison on your page?
I have indeed started the development when I found that connexion does not completely meet my requirements. The main difference is that pyotr supports ASGI, so can work with any ASGI framework (I haven't tried it with Mangum yet, but am planning to). Additionally, pyotr has a client component, which was inspired by Bravado.