It looks like at least on my Ubuntu 16.04, pip is shared for all Python versions in /usr/lib/python3/dist-packages/pip.
This is what I did to get it working again:
sudo apt remove python3-pipsudo python3.8 -m easy_install pip
You might want to install the python 3.5 version of pip again with sudo python3.5 -m easy_install pip.