update get-pip.py location

[k-okada]

latest get-pip.py does not work on Python 2.7

This script does not work on Python 2.7 The minimum supported Python version is 3.6. Please use https://bootstrap.pypa.io/pip/2.7/get-pip.py instead.

use https://bootstrap.pypa.io/pip/2.7/get-pip.py, instead of https://bootstrap.pypa.io/get-pip.py