sabnzbd: stay with sabyenc3 4.0.0

SABnzbd requires `sabyenc3` at version `4.0.0`, but it was updated to
`5.0.1`.

This fixes the error:

```
SABYenc disabled: no correct version found! (Found v5.0.1, expecting
v4.0.0)
```

pkgs/servers/sabnzbd/default.nix

@@ -10,7 +10,21 @@
 }:
 
 let
-  pythonEnv = python3.withPackages(ps: with ps; [
+
+  python = python3.override {
+    packageOverrides = final: prev: {
+      sabyenc3 = prev.sabyenc3.overridePythonAttrs (oldAttrs: rec {
+        version = "4.0.0";
+        src = oldAttrs.src.override {
+          inherit version;
+          hash = "sha256-PwwQ2jChKIqh7jJ6E2hkqPquTDSN4MklghfJ+MkM0n0=";
+        };
+      });
+    };
+    self = python;
+  };
+
+  pythonEnv = python.withPackages(ps: with ps; [
     chardet
     cheetah3
     cherrypy