Say I have created a python project foobar with the following layout:

package support multiple version of python 2.7 and 3.9. Need to use the pyproject.toml for package.
How to specify a different requirement file based on python version ?

I have a main Python package, foo, and want to have a number of separately maintained subpackages foo.bar etc. The subpackages only contain data, i.e. yaml files, which are located in the root of my (git) repository. What needs to be in the pyproject.toml to achieve this? My minimal start was