Use a src/-style structure

[qwenger]

As shown in the guide: https://packaging.python.org/en/latest/tutorials/packaging-projects/

While this is not a hard requirement, the post https://ianhopkinson.org.uk/2022/02/understanding-setup-py-setup-cfg-and-pyproject-toml-in-python/ mentions a good reason to do it: it ensures that the package has been built/installed before testing it.