pythonInterpreters: get self from __splicedPackages

BEFORE:

the python derivation did not contain .nativeDrv and .crossDrv because
it was not from the __splicedPackages set

the python used in mk-python-derivation.nix was for host

```
nix-repl> pkgsCross.aarch64-multiplatform.python3Packages.xpybutil.nativeBuildInputs
[ «derivation /nix/store/bhz39ds4v02hn6x4py4mzjyilw4a589h-python3-aarch64-unknown-linux-gnu-3.10.7.drv» «derivation /nix/store/v880cnh4ml7czmivfbk3cdh93hz9yvbn-hook.drv» «derivation /nix/store/f243ab7wv92gqsmc9h7gr0qcnj0xcgdb-hook.drv» «derivation /nix/store/880lf8895bzn8d94lrr2y7ilgkxq0lc4-python-remove-tests-dir-hook.drv» «derivation /nix/store/fzjnhawfs1wpw58hcd1vxd9y750dc08y-python-remove-bin-bytecode-hook.drv» «derivation /nix/store/n8l59iparx98yfw8g5ydqmzmk3fdic75-setuptools-setup-hook.drv» «deri→
```

AFTER:

the python derivation does contain .nativeDrv and .crossDrv because
it is from the __splicedPackages set

those 2 are what makes nativeBuildInputs and buildInputs function
properly

the python used in mk-python-derivation.nix is for build

```
nix-repl> pkgsCross.aarch64-multiplatform.python3Packages.xpybutil.nativeBuildInputs
[ «derivation /nix/store/hvb9yxgv1133cfhxxd869sibldvv2vdx-python3-3.10.7.drv» «derivation /nix/store/v880cnh4ml7czmivfbk3cdh93hz9yvbn-hook.drv» «derivation /nix/store/f243ab7wv92gqsmc9h7gr0qcnj0xcgdb-hook.drv» «derivation /nix/store/880lf8895bzn8d94lrr2y7ilgkxq0lc4-python-remove-tests-dir-hook.drv» «derivation /nix/store/fzjnhawfs1wpw58hcd1vxd9y750dc08y-python-remove-bin-bytecode-hook.drv» «derivation /nix/store/n8l59iparx98yfw8g5ydqmzmk3fdic75-setuptools-setup-hook.drv» «derivation /nix/store/7vyhynla→
```

pkgs/development/interpreters/python/default.nix

···
-{ pkgs
+{ __splicedPackages
 , callPackage
 , config
 , darwin
···
 in {
 
   python27 = callPackage ./cpython/2.7 {
-    self = pkgs.python27;
+    self = __splicedPackages.python27;
     sourceVersion = {
       major = "2";
       minor = "7";
···
   };
 
   python37 = callPackage ./cpython {
-    self = pkgs.python37;
+    self = __splicedPackages.python37;
     sourceVersion = {
       major = "3";
       minor = "7";
···
   };
 
   python38 = callPackage ./cpython {
-    self = pkgs.python38;
+    self = __splicedPackages.python38;
     sourceVersion = {
       major = "3";
       minor = "8";
···
   };
 
   python39 = callPackage ./cpython ({
-    self = pkgs.python39;
+    self = __splicedPackages.python39;
     inherit (darwin) configd;
     inherit passthruFun;
   } // sources.python39);
 
   python310 = callPackage ./cpython ({
-    self = pkgs.python310;
+    self = __splicedPackages.python310;
     inherit (darwin) configd;
     inherit passthruFun;
   } // sources.python310);
 
   python311 = callPackage ./cpython {
-    self = pkgs.python311;
+    self = __splicedPackages.python311;
     sourceVersion = {
       major = "3";
       minor = "11";
···
 
   # Minimal versions of Python (built without optional dependencies)
   python3Minimal = (callPackage ./cpython ({
-    self = pkgs.python3Minimal;
+    self = __splicedPackages.python3Minimal;
     inherit passthruFun;
     pythonAttr = "python3Minimal";
     # strip down that python version as much as possible
···
   });
 
   pypy27 = callPackage ./pypy {
-    self = pkgs.pypy27;
+    self = __splicedPackages.pypy27;
     sourceVersion = {
       major = "7";
       minor = "3";
···
     sha256 = "sha256-wERP2YcwWMHA2Z4TqTTpIoXLBZksmWi/Ujwyv5vsCp0=";
     pythonVersion = "2.7";
     db = db.override { dbmSupport = !stdenv.isDarwin; };
-    python = pkgs.python27;
+    python = __splicedPackages.python27;
     inherit passthruFun;
     inherit (darwin) libunwind;
     inherit (darwin.apple_sdk.frameworks) Security;
   };
 
   pypy38 = callPackage ./pypy {
-    self = pkgs.pypy38;
+    self = __splicedPackages.pypy38;
     sourceVersion = {
       major = "7";
       minor = "3";
···
     sha256 = "sha256-Ia4zn09QFtbKcwAwXz47VUNzg1yzw5qQQf4w5oEcgMY=";
     pythonVersion = "3.8";
     db = db.override { dbmSupport = !stdenv.isDarwin; };
-    python = pkgs.python27;
+    python = __splicedPackages.python27;
     inherit passthruFun;
     inherit (darwin) libunwind;
     inherit (darwin.apple_sdk.frameworks) Security;
   };
-  pypy37 = pkgs.pypy38.override {
-    self = pkgs.pythonInterpreters.pypy37;
+  pypy37 = __splicedPackages.pypy38.override {
+    self = __splicedPackages.pythonInterpreters.pypy37;
     pythonVersion = "3.7";
     sha256 = "sha256-LtAqyecQhZxBvILer7CGGXkruaJ+6qFnbHQe3t0hTdc=";
   };
 
   pypy27_prebuilt = callPackage ./pypy/prebuilt_2_7.nix {
     # Not included at top-level
-    self = pkgs.pythonInterpreters.pypy27_prebuilt;
+    self = __splicedPackages.pythonInterpreters.pypy27_prebuilt;
     sourceVersion = {
       major = "7";
       minor = "3";
···
 
   pypy38_prebuilt = callPackage ./pypy/prebuilt.nix {
     # Not included at top-level
-    self = pkgs.pythonInterpreters.pypy38_prebuilt;
+    self = __splicedPackages.pythonInterpreters.pypy38_prebuilt;
     sourceVersion = {
       major = "7";
       minor = "3";