stdenv: bootstrap darwin with python3

- Replaced python override from the final stdenv, instead we
  propagate our bootstrap python to stage4 and override both
  CF and xnu to use it.

- Removed CF argument from python interpreters, this is redundant
  since it's not overidden anymore.

- Inherit CF from stage4, making it the same as the stdenv.

pkgs/development/interpreters/python/cpython/2.7/default.nix

@@ -11,7 +11,7 @@
 , tcl ? null, tk ? null, tix ? null, xlibsWrapper ? null, libX11 ? null, x11Support ? false
 , zlib
 , self
-, CF, configd, coreutils
+, configd, coreutils
 , python-setup-hook
 # Some proprietary libs assume UCS2 unicode, especially on darwin :(
 , ucsEncoding ? 4
@@ -180,7 +180,7 @@ let
     ++ optional stdenv.hostPlatform.isCygwin expat
     ++ [ db gdbm ncurses sqlite readline ]
     ++ optionals x11Support [ tcl tk xlibsWrapper libX11 ]
-    ++ optionals stdenv.isDarwin ([ CF ] ++ optional (configd != null) configd);
+    ++ optional (stdenv.isDarwin && configd != null) configd;
   nativeBuildInputs =
     optionals (stdenv.hostPlatform != stdenv.buildPlatform)
     [ buildPackages.stdenv.cc buildPackages.python ];