Including data files in a package

[snippsat]

but all the information I have found on the Web states that for a binary package I need to use setup.py.

To do most setup from setup.cfg is relative new,so most tutorial/demo you find on web will be for setup.py.
Can still use setup.py it will work fine or can rewrite it to use setup.cfg
Look at Configuring setup() using setup.cfg files
A sample Python project

And look my answer in your last post.
in setup.cfg to add files eg .dat use [options.data_files](same as package_data).

In link you see for setup.py is this way.

# If there are data files included in your packages that need to be
# installed, specify them here.
package_data={  # Optional
        'sample': ['package_data.dat'],
    },