getting `pip` to use `python3` instead of macOS-supplied python2?
There is no way to overwrite the existing System python(s) with newer versions. (Catalina comes with a version of python3.)

The command pip3 will specify that you want to install modules to python3. 

python2 will of course continue to work after 1st Jan, and many people still have lots of scripts that they will continue to run.