My first site using Flask and hosted on heroku used bootstrap 4.1 (I didn't use 5 because only 4.1 documentation was translated to my language)
git clone https://github.com/ward910/PYTman.git
cd PYTman
pip install -r requirements.txt
export FLASK_ENV=development
export FLASK_APP=main.py
flask run
or
python main.py // No debug