Pip for Python 3.8

sudo apt install python3.8
sudo apt install python3.8-distutils

wget https://bootstrap.pypa.io/get-pip.py
sudo python3.8 get-pip.py

Install pip the official way:

curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
python3.8 get-pip.py

made 3.8 my default Python version

It depends on how you did that, but it might break something in your OS. For example some packages on Ubuntu 18.04 might depend on python being python2.7 or python3 being python3.6 with some pip packages preinstalled.