pytest strongly recommend this https://docs.pytest.org/en/latest/goodpractices.html and point to https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-structure for more benefits.
See also:
https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/
https://www.pyopensci.org/python-package-guide/package-structure-code/python-package-structure.html
https://www.b-list.org/weblog/2023/dec/15/python-packaging-src-layout/