fix: use pip3 instead of pip

[JavierCane]

• We are actually checking if platform::command_exists pip3 (not platform::command_exists pip before executing pip::update_all
• Inside the pip::update_all function we are already interacting with pip3 in order to list outdated dependencies and show information about them
• It does not feel consistent to use pip while actually updating these dependencies
• It even throws an error every time we execute the up dotly command if we have not installed a Python environment further more the one which comes with macOS by default