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 https://github.com/nix-community/poetry2nix/commit/5612b5cbdf6b1c532165478761f840d7cbfda37f.