I use pip-compile from pip-tools to create a requirements.txt which has all dependencies (including all transitive ones) which fulfill all requirements and are consistent. This is nice for applications which I run via CI/CD.
For libraries, I think it makes sense to create a requirements.txt which has all of the oldest dependencies. This is what I would like to run the test against, because there it is most likely that I depend on a later version and I might not be aware of it.
Does pip-compile support this?
As far as I know this is not yet possible but I raised a feature request at https://github.com/jazzband/pip-tools/issues/1279 for adding it.
There is a very good use-case for this as I often introduced bugs my mistake by starting to use features that required a dependency version bump.