• Closes "Package would be ignored" warnings when building the pvlib distribution #1474
• I am familiar with the contributing guidelines
• [ ] Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
• [ ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

According to people who keep better track than I do of the evolution of python packaging over time, any folder is now considered a python package, even if it doesn't have any python-related files (see pypa/setuptools#3340 (comment)). This prompted some deprecations in setuptools, leading to the warnings in #1474. According to pypa/setuptools#3340 (comment) and similar posts elsewhere, using setuptools.find_namespace_packages() is a recommended fix.

In our case find_namespace_packages() includes a bunch of other stuff we don't actually want, but find_packages() seems closer to the mark. Making that change seems to get rid of the warnings without changing which files get included in the sdist:

# use `cut` to remove the first component of filepaths, which we don't care about here
$ tar tf pvlib-0.9.1+12.gc8b04a56.tar.gz | cut -d / -f 2- > old.txt  # current master
$ tar tf pvlib-0.9.1+18.g071cc215.tar.gz  | cut -d / -f 2- > new.txt  # current HEAD on this branch
$ diff old.txt new.txt  # no output (except unrelated changes, e.g. new pvfactors gallery example file), so list of filenames did not change

I don't really understand what's happening under the hood here, but I want to see what the CI thinks of this change. Keeping as a draft until I have a better understanding of what setuptools is doing.