Do not propagate pytest

[SuperSandro2000]

See NixOS/nixpkgs#115564 (comment)

pytest plugins/extensions should not propagate pytest, see c44ae94

[FRidh]

Note this really is just an ugly work-around. To be proper, one would have to override the package set from within itself. In case of test-dependencies that would be fine (does cause some rebuilds), but showing that possibility will mean overriding of run-time dependencies is bound to happen as well, and that cannot happen because it would result in conflicts.