test: drop -k from -k stages/test test matrix

With pytest 8.0.0 the `-k` option seem to have changed it's
behavior. Drop `-k` therefore.

Tests started to fail recently and it looks like this is because
pytest 8.0.0 changes the semantic of the `-k` option. We used
to pass `-k stages/test` but that seems to no longer work. So
pin pytest to the last good version until this is better
understood.

.github/workflows/test.yml

@@ -19,7 +19,7 @@ jobs:
         - "test.run.test_noop"
         - "test.run.test_sources"
         - "test.run.test_stages"
-        - "-k stages/test"
+        - "stages/test"
         environment:
         - "py36"
         - "py39"