[requirements][sdk] Requirements freeze exception for 'fixtures'

o/ I would like to request a requirements freeze exception to bump `fixtures` from 4.3.0 to 4.3.1, as proposed here [1]. As with my earlier testtools request, fixtures 4.3.1 includes a small type hint fix [2] that resolves pep8 failures currently seen in the gates of a number of SDK deliverables. Once again, the bump carries effectively zero risk and bumping it would be both easier and safer than updating the affected projects across both master and stable/2026.1 to add `type: ignore` comments or disable the check. Cheers, Stephen [1] https://review.opendev.org/c/openstack/requirements/+/978871 [2] https://github.com/testing-cabal/fixtures/compare/4.3.0...4.3.1