That's what it told me to do, and indeed I did that.
Usually it is not recommended to do that.
Such problems can appear when mixing distribution manager (apt-get) and other tools to update software (in this case pip itself).
Did it say that there is a new version and you
should update or that it is mandatory ?
Anyway, i would try to uninstall it and reinstall the package-manager version of pip, and then try it again without updating pip:
sudo python3 -m pip uninstall pip && sudo apt-get install python3-pip --reinstall
Using
python3 -m pip install -r requirements.txt should work afterwards.
By the way, which version of python3 and pip are you using?
python3 --version && pip3 --version
Do you have Python 3.6? I completely forgot it was only available for that specific version of Python