pip has dropped support for Python 2 and 3.5. You will need to use a version-specific branch, assuming that your Python version is 3.5:
curl -fsSL https://bootstrap.pypa.io/pip/3.5/get-pip.py | python3.5
get-pip.py reference: https://pip.pypa.io/en/stable/installing/#installing-with-get-pip-py
Related: How can I install a legacy PIP version with python 2.6.6 or python 2.7.5?
Better yet, as suggested in the comments, install a newer version of Python that is not end-of-life.