Revert "Revert "python3: splice python within python3Packages.callPackage""

This reverts commit 2ffca30cde.

`793cc9d982415b71cdba729cf779bfc49e9d2ae7` `python3: splice python within python3Packages.callPackage`
Was reverted because it broke
`pkgsCross.aarch64-multiplatform.python3Packages.cryptography`

But by reverting the revert `pkgsCross.aarch64-multiplatform.python3Packages.cryptography` still builds.

I tried reverting the 2 cryptography update commits, and it still
worked, so I do not know why this would work now.

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;