This is to avoid subtle behaviour that contributed to root cause of
https://github.com/pelican-plugins/sitemap/issues/36
Specifically: if installing Pelican into a local virtualenv with the pdm or uv
default name ".venv", then subdirectories of .venv will contain all of the test
.rst and .md files.
If you then run Pelican in that same root directory using the default PATH
value (".") then it will add those content files to the site.