pip: no module named _internal

I have a problem when I try to use pip in any way. I'm using Ubuntu 16.04.4 I should say that I've used it already, and I never had any problem, but starting today when I use any command I always get the same error (as an example using `pip --upgrade`). ``` Traceback (most recent call last): File "/usr/local/bin/pip", line 7, in <module> from pip._internal import main ImportError: No module named _internal ``` I have tried doing `sudo apt-get remove python-pip` followed by `sudo apt-get install python-pip` but nothing changed.