Fix handling of file inclusion

[znicholls]

At the moment we have files specified in MANIFEST.IN and in setup.py, there must be a better way to do this.

[rgieseke]

Climate Categories only seems to have this in setup.cfg:

https://github.com/pik-primap/climate_categories/blob/main/setup.cfg#L70

[znicholls]

Ye I think I'm going to have to switch to toml files etc.

[rgieseke]

Not sure if that's necessary ...

https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

[rgieseke]

Maybe it's still not solved for binary/source packages ...

https://stackoverflow.com/a/14159430