from pip._internal.cli.main import main code example

Example 1: ModuleNotFoundError: No module named 'pip._internal.cli.main'

python3 -m pip install --upgrade pip

Example 2: from pip import main ImportError: cannot import name main

You must have inadvertently upgraded your system pip (probably through 
something like sudo pip install pip --upgrade)

pip 10.x adjusts where its internals are situated. The pip3 command 
you're seeing is one provided by your package maintainer 
(presumably debian based here?) and is not a file managed by pip.

You can read more about this on pip's issue tracker

You'll probably want to not upgrade your system pip and instead 
use a virtualenv.

To recover the pip3 binary you'll need to sudo python3 -m pip 
uninstall pip && sudo apt install python3-pip --reinstall.

If you want to continue in "unsupported territory" (upgrading a 
system package outside of the system package manager), you can probably
get away with python3 -m pip ... instead of pip3.