Fix Readme for post PEP 668 installation

[alecmalloc]

Just a heads up,

most users on a newer system will run into issues using:
python3 -m pip install --upgrade pip setuptools
python3 -m pip install norminette

syntax post PEP 668 looks like:
sudo apt install python3-setuptools
sudo apt install pipx
pipx install norminette

If you'd like I can open a PR to add to readme ;)

[matthieu42Network]

Hello, We'll be glad if you do a PR for this :)