> by having a single monolith you now have an implicit dependency on the same runtime working on all parts of your code

I think many people would not consider what I am going to suggest a monolith, but docker compose all hosted in the same server acts a lot like a monolith.

> particularly true in Python, where it's not a common/well-supported use case to have multiple versions of library dependencies

You can use virtual environments each with its python version and packages.