First of all you have to be sure that you've installed Python3, and have support for virtualenvironments for Python.
Install Python3:
$ sudo apt-get update
$ sudo apt-get install -y python3.6
Install pip, virtualenv:
$ sudo apt-get install -y python3-pip
$ sudo apt-get install -y python3-venv
Create a virtualenvironment to install python packages, you can put it to ~/.env/LF/
$ mkdir ~/.env/
$ python3 -m venv ~/.env/
Change your environment to newly created one. Before running development server you have to change your environment too:
$ source ~/.env/LF/bin/activate
Now you can install requirements:
$ pip3 install -r requirements.txt
And run the development server:
$ hug -f main.py