BuildStream doesn't fix the version of its dependencies

Summary

BuildStream is installing a broken version of ruamel.yaml.

Steps to reproduce

See defs2bst log: https://gitlab.com/baserock/definitions/-/jobs/85875464

What is the current bug behavior?

When installing BuildStream there is not fixed version of the dependencies to install (see setup.py). Making possible to install some broken dependencies. Sometimes some projects will change their APIs which will also result in a broken BuildStream.

What is the expected correct behavior? (Possible fixes)

We should fix the versions of the dependencies, so that if in a year I come back and try to use the same version of BuildStream, I have a working tool, and the same results.

• BuildStream version affected: all (master, 1.0.1, 1.1.3, etc)

---