tools/qa-checks: Install pip for Python 3.6

The python-pip from from https://bootstrap.pypa.io/get-pip.py does not support
Python 3.6 anymore. We should install it from https://bootstrap.pypa.io/pip/3.6/get-pip.py


instead.

Issue-Id: SCM-4067
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: Idc432466e1e7c6735df306c4f048e6c036b40f4b