4 test failures with fixtures>=4.0.0

Here is a link to the relevant logs: https://paste.opendev.org/show/815627/

This happens with fixtures 4.0.1 and fixtures 4.0.0. This does not happen with fixtures 3.0.0.

I bisected the issue and found the commit that introduces the issue is ed788ce76f0846795dd68f487958b743922090aa (see https://github.com/testing-cabal/fixtures/commit/ed788ce76f0846795dd68f487958b743922090aa). I have no idea why it breaks oslo.config's tests.