Fix manifest data specifier

[rtburns-jpl]

include matches files relative to the project, so this include pattern did nothing We need to use recursive-include to match files with a specific file extension under a given directory

See https://setuptools.pypa.io/en/latest/userguide/miscellaneous.html#controlling-files-in-the-distribution

[scottstanie]

Agh, this still took me 3 reads of their page to understand. Thanks Ryan!