My shell scripts I use all the time, available on Github so I can pull them down quickly to multiple machines

After you install Python3 with Homebrew on a fresh install of macOS, trying to use pip3 results in an error like this:

× This environment is externally managed
╰─> To install Python packages system-wide, try brew install
    xyz, where xyz is the package you are trying to
    install.
    
    If you wish to install a Python library that isn't in Homebrew,
    use a virtual environment:

    ...

And on and on. So the solution is to symlink the simple scripts mython3 and mip3 into ~/bin/ which should be on your path, then run this command twice:

$ python3 -m venv ~/pyvenv

Which will make a user-specific virtual environment in ~/pyvenv

Note that for some reason you must run the command twice to get the activate script to appear so the mython3 and mip3 scritps work.

After that always use mython3 and mip3 intead of python3 and pip3 to have access to user-installed packages, installed with mip3 install xyz

Actually, in actual Python scripts, just use

#!/Users//pyvenv/bin/python3

and it will use your packages, installed with mip3 (mython3 cannot be used in #! in Pythons scripts for whatever reason).