Prevent data files from being included in GitHub release assets

[kafitzgerald]

Currently, everything in the repo is included in the GitHub release assets. Given the number of data files in here, that's probably not optimal (especially since we pull this in for conda-forge releases).

On the conda-forge end, I think we can pull from the PyPI sdist instead, but I think changing this up is still worth some thought.

@anissa111, kindly pointed me here which looks promising: https://git-scm.com/docs/gitattributes#_export_ignore

[kafitzgerald]

I think my only potential concern here with moving ahead on this is that there may be use cases where a full repo archive might be helpful (e.g. for folks with connectivity issues). And given that we don't need to do all that many releases here anyway, maybe it's not really that big of a deal to have the releases on here include everything.

Anyway, something to think on.