Overall, this looks fine as-is. I've added a couple tiny nits, but you can take them or leave them.

In src/tests/conftest.py, we do have a backend fixture that will iterate over the backends for the current platform. This can be used explicitly by a test that needs to iterate over the backends. It can also be used implicitly via the mss_impl fixture, which gives you an MSS factory, iterating over the backends. Both of these mean that tests that use those will run multiple times, once for each backend. When we do add the dxgi backend, you should fix the backend fixture accordingly.