Oldest supported versions of dependencies

[hombit]

Feature request

I propose setting (in pyproject.toml), documenting, and running CI with the oldest supported versions of all our dependencies.

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

[hombit]

Related PPT issue: lincc-frameworks/python-project-template#495