Yes, Snakemake [requires python >= 3.5](https://bitbucket.org/snakemake/snakemake/src/4e41054b509aa0a0913c708e859b50833aa08769/setup.py?at=v5.2.2&fileviewer=file-view-default#setup.py-12).  However, it's makes sense set the python version at `top-level/all-packages.nix`. Done.