Add docs requirements as data file

[smokestacklightnin]

This fixes a build error by including the requrements-docs.txt file as a data file

[peytondmurray]

Did you check whether this works? I don't think it's actually putting the requirements file in the right place. Is it possible that setuptools isn't finding it because it's in the root of the repo? https://setuptools.pypa.io/en/latest/userguide/datafiles.html#package-data

[smokestacklightnin]

Did you check whether this works?

I actually don't believe it does. Closing this PR as a consequence.

Is it possible that setuptools isn't finding it because it's in the root of the repo?

This is what I believe to be the case. I also firmly believe that requirements-docs.txt should stay where it is located in the directory tree, rather than being inside the Python module, so this solution could never work.