Use upper-constraints from in review changes

Currently upper-constraints.txt is not getting used
from in-review changes of requirements project and
leading to merge of broken requirements[1].

Use master branch to fetch constraints instead of
the remote branch.

[1] https://review.opendev.org/c/openstack/requirements/+/822575

Depends-On: https://review.opendev.org/c/openstack/requirements/+/823128
Change-Id: I5d42ac6b54bf20804d7e5faa39d1289102318b64
1 file changed