pythonInterpreters: get self from __splicedPackages
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→
```

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs