Thanks for your reply. No I didn’t get an error because I didn’t know it was possible to install Druid on RPi, I might have missed that in the docs, is Python pre-installed on Stretch or do I I need to install it before installing Druid ? I will try again tomorrow. And about the Mac, I’ve tried to install Homebrew on this old machine but without success, I’ll try with an older version 
Edit
You’re right, on Raspbian I need to run Druid with ~/.local/bin/druid
This is the install process I used, in case it’s useful for anyone:
Python 2.7 and 3.5 come both pre-installed on Raspbian Stretch, however by default it will use 2.7, so:
sudo nano ~/.bashrc and add the following line at the end of the file.
alias python='/usr/bin/python3'
save and reboot
Now python --version returns Python 3.5.3
sudo apt-get install python3-prompt-toolkit
sudo apt-get install python3-pip
pip3 install --upgrade setuptools
pip3 install monome-druid
Run Druid ~/.local/bin/druid