It seems that the poetry2nix
Mypy problem is documented in mypy patch is failing · Issue #561 · nix-community/poetry2nix · GitHub, and it seems to have been fixed in Nixpkgs unstable. But I just updated my NixOS unstable, and got the same error message for both Mypy 0.940 and 0.942.
This is because I’m using wheels, which doesn’t require patching. Fixed in mypy: Don't patch if format is wheel · nix-community/poetry2nix@5612b5c · GitHub.