python3: splice python within python3Packages.callPackage

`lib.extends hooks pythonPackagesFun` includes python which we want to
splice

what this fixes:
```
nix-repl> lib.elemAt pkgsCross.aarch64-multiplatform.python3Packages.xpybutil.nativeBuildInputs 0
«derivation /nix/store/39dkb51rciw6zwg0c2c44gpmpjapddxc-python3-aarch64-unknown-linux-gnu-3.10.12.drv»
```
to
```
nix-repl> lib.elemAt pkgsCross.aarch64-multiplatform.python3Packages.xpybutil.nativeBuildInputs 0
«derivation /nix/store/21ldw2dp26xvv9iyxn9x77a8yh4waqz5-python3-3.10.12.drv»
```

Before #194205 The keep python used to work by accident because self was passed from
__splicedPackages https://github.com/NixOS/nixpkgs/pull/196052/files#diff-44ce3495c4f983ce64dd47c86a9d3e77bad210b2709c098a3806998dcd9b000bR213
But now it does not work because overrideAttrs is used on python in
python-packages-base.nix

Preferably we would [ splice the hooks too ](#228139) but we cannot do that until [Package sets within derivations (i.e. python3.pkgs) are not spliced](#211340) is fixed, because people often use `python3.pkgs.wrapPython` in `nativeBuildInputs` (it's correct but python3.pkgs should be python3Packages to get splicing.

pkgs/development/interpreters/python/hooks/default.nix

@@ -1,9 +1,9 @@
-self: super: with self;
+self: dontUse: with self;
 
 let
-  pythonInterpreter = super.python.pythonForBuild.interpreter;
-  pythonSitePackages = super.python.sitePackages;
-  pythonCheckInterpreter = super.python.interpreter;
+  pythonInterpreter = python.pythonForBuild.interpreter;
+  pythonSitePackages = python.sitePackages;
+  pythonCheckInterpreter = python.interpreter;
   setuppy = ../run_setup.py;
 in {
   makePythonHook = args: pkgs.makeSetupHook ({passthru.provides.setupHook = true; } // args);

pkgs/development/interpreters/python/passthrufun.nix

@@ -47,12 +47,13 @@
         selfTargetTarget = pythonOnTargetForTarget.pkgs or {}; # There is no Python TargetTarget.
       };
       hooks = import ./hooks/default.nix;
-      keep = lib.extends hooks pythonPackagesFun;
+      keep = self: hooks self {};
       extra = _: {};
       optionalExtensions = cond: as: lib.optionals cond as;
       pythonExtension = import ../../../top-level/python-packages.nix;
       python2Extension = import ../../../top-level/python2-packages.nix;
       extensions = lib.composeManyExtensions ([
+        hooks
         pythonExtension
       ] ++ (optionalExtensions (!self.isPy3k) [
         python2Extension
@@ -64,7 +65,7 @@
       otherSplices
       keep
       extra
-      (lib.extends (lib.composeExtensions aliases extensions) keep))
+      (lib.extends (lib.composeExtensions aliases extensions) pythonPackagesFun))
     {
       overrides = packageOverrides;
       python = self;