Fix "global name 'blacklist' is not defined" bug.
Fix "global name 'blacklist' is not defined" bug.

Correct backlist to blacklist in project-requirements-change.py.
Refer to https://review.openstack.org/#/c/473402 for more info.

Change-Id: I392737bc10752a9bfa23e985763dd5b6e7d19954

openstack-infra/project-config,openstack-infra/project-config