The setup_pip.sh script previously used PIP_CONSTRAINT, which is also
an environment variable recognized by pip for constraint files.
This caused pip to misinterpret version specifiers like "<25.3"
as a path to a requirements file, resulting in installation errors.
Change-Id: I63e95028cda5a9ded8d7391abbb272f70834f50d
Signed-off-by: Artem Vasilyev <artem.v.vasilyev@gmail.com>