doc/python: simplify packageOverrides example

doc/languages-frameworks/python.section.md

@@ -293,29 +293,23 @@ the overrides for packages in the package set.
 ```nix
 with import <nixpkgs> { };
 
-(
-  let
-    python =
-      let
-        packageOverrides = self: super: {
-          pandas = super.pandas.overridePythonAttrs (old: rec {
-            version = "0.19.1";
-            src = fetchPypi {
-              pname = "pandas";
-              inherit version;
-              hash = "sha256-JQn+rtpy/OA2deLszSKEuxyttqBzcAil50H+JDHUdCE=";
-            };
-          });
-        };
-      in
-      pkgs.python3.override {
-        inherit packageOverrides;
-        self = python;
-      };
-
-  in
-  python.withPackages (ps: [ ps.blaze ])
-).env
+let
+  python = pkgs.python3.override {
+    packageOverrides = self: super: {
+      pandas = super.pandas.overridePythonAttrs (
+        finalAttrs: prevAttrs: {
+          version = "0.19.1";
+          src = fetchPypi {
+            pname = "pandas";
+            inherit (finalAttrs) version;
+            hash = "sha256-JQn+rtpy/OA2deLszSKEuxyttqBzcAil50H+JDHUdCE=";
+          };
+        }
+      );
+    };
+  };
+in
+(python.withPackages (ps: [ ps.blaze ])).env
 ```
 
 The next example shows a non trivial overriding of the `blas` implementation to