doc/python: use python3Packages instead of python3.pkgs

Allows splicing to work correctly

from issue 211340

>> 3: Discourage using package sets like python3.pkgs "directly"

> I think about 8 years ago I introduced this and it has been a clear mistake. Since then this pattern was copied throughout. We should indeed go to pythonPackages and also move the helpers such as buildEnv and withPackages into the package set, like haskell already had before.

- FRidh

doc/languages-frameworks/python.section.md

@@ -282,11 +282,11 @@ specifying an interpreter version), like this:
 
 ```nix
 { lib
-, python3
+, python3Packages
 , fetchPypi
 }:
 
-python3.pkgs.buildPythonApplication rec {
+python3Packages.buildPythonApplication rec {
   pname = "luigi";
   version = "2.7.9";
   pyproject = true;
@@ -297,13 +297,13 @@ python3.pkgs.buildPythonApplication rec {
   };
 
   nativeBuildInputs = [
-    python3.pkgs.setuptools
-    python3.pkgs.wheel
+    python3Packages.setuptools
+    python3Packages.wheel
   ];
 
-  propagatedBuildInputs = with python3.pkgs; [
-    tornado
-    python-daemon
+  propagatedBuildInputs = [
+    python3Packages.tornado
+    python3Packages.python-daemon
   ];
 
   meta = with lib; {