Revert "python3Packages.rx: 3.2.0 -> 4.0.4"
Revert "python3Packages.rx: 3.2.0 -> 4.0.4"

https://github.com/ReactiveX/rxpy renamed the module name to `reactivex`
effectively breaking dependent packages.

This reverts commit eb885d30f983a78778220b10309f6d10b1295cde.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs