Hi Allen,
Harvest 1.6 is based on Perl, the Python packages dependencies are required for a few extensions that are written in Python3. You should indeed use pip3 for those! For more details you can look at the documentation that comes with the package. Otherwise let me know if you have specific issues, I'll try to help (unfortunately I don't always find the time for community...).
One more thing: we have stopped developing the Perl source-code and have recently released an Alpha version of Harvest 2.0 which is completely in Python3 (with the possibility of moving some parts to Go in the future). This new Harvest supports more databases: Prometheus and InfluxDB in addition to Graphite/Whisper. It doesn't not support 7Modes yet, but that will be added in the Beta released which is scheduled this month.