libxml2: use python3 for build and as default for bindings

Changing the default may cause breakage, however, users should have
already switched to `pythonPackages.libxml2` long ago.

pkgs/development/libraries/libxml2/default.nix

@@ -1,15 +1,12 @@
 { stdenv, lib, fetchurl
-, zlib, xz, python2, ncurses, findXMLCatalogs
+, zlib, xz, python, ncurses, findXMLCatalogs
 , pythonSupport ? stdenv.buildPlatform == stdenv.hostPlatform
 , icuSupport ? false, icu ? null
 , enableShared ? stdenv.hostPlatform.libc != "msvcrt"
 , enableStatic ? !enableShared,
 }:
 
-let
-  python = python2;
-
-in stdenv.mkDerivation rec {
+stdenv.mkDerivation rec {
   pname = "libxml2";
   version = "2.9.9";
 

pkgs/top-level/all-packages.nix

@@ -12723,7 +12723,9 @@ in
 
   libxmi = callPackage ../development/libraries/libxmi { };
 
-  libxml2 = callPackage ../development/libraries/libxml2 { };
+  libxml2 = callPackage ../development/libraries/libxml2 {
+    python = if stdenv.isDarwin then python2 else python3;
+  };
 
   libxml2Python = pkgs.buildEnv { # slightly hacky
     name = "libxml2+py-${res.libxml2.version}";

pkgs/top-level/python-packages.nix

@@ -3689,7 +3689,7 @@ in {
   libplist = disabledIf isPy3k
     (toPythonModule (pkgs.libplist.override{python2Packages=self; })).py;
 
-  libxml2 = toPythonModule (pkgs.libxml2.override{pythonSupport=true; python2=python;}).py;
+  libxml2 = toPythonModule (pkgs.libxml2.override{pythonSupport=true; inherit python;}).py;
 
   libxslt = disabledIf isPy3k
     (toPythonModule (pkgs.libxslt.override{pythonSupport=true; python2=python; inherit (self) libxml2;})).py;