That's actually really easy. Your target here is your venv site-packages, while requirements file is the prerequisite. The idea is that your site-packages folder has to be recreated/updated (pip install) when it is older than the requirements file. One thing you have to do is to simply update the directory timestamp each time you add a dependency, because adding a file into a folder does not change the folder timestamp. Having that it would be like

venv/site-packages: requirements.txt pip install -r $< touch $@