So, I wanted to play with a python package, but I didn’t want to install it into my primary python distribution tree just yet. I couldn’t find out how to override “site-packages”, though.

Finally, I found this:

Create a file called “setup.cfg” in the directory that has your “setup.py”. In it, put:
[install]
install_lib = /var/tmp/test-blort/lib
install_scripts = /var/tmp/test-blort/bin

Run your setup.py, and away you go.