python2 python3 messed up : How to fix ?

[Gribouillis]

I don't know this python-is-python3 package that you installed. Obviously it changed your default python command from python 2 to python 3, which is good, but it should also have changed the default pip command to work for python 3.

It means that when a site tells you to use the pip command you should use python -m pip instead or perhaps pip3 or even python3.8 -m pip as we did here to make sure that the package is installed for python 3 instead of python 2.

The problem in your case is that the command also installed another command platformio and this command uses python. As you installed it for python 2, it was unable to handle the fact that the python command now runs a different interpreter.