buildPythonPackage: introduce "disabled" argument

Useful to disable the package for specific python versions.

Typically usage: disable = isPy3k;

pkgs/development/python-modules/generic/default.nix

@@ -34,7 +34,10 @@
 # The difference is that `pythonPath' is not propagated to the user
 # environment.  This is preferrable for programs because it doesn't
 # pollute the user environment.
-,  pythonPath ? []
+, pythonPath ? []
+
+# used to disable derivation, useful for specific python versions
+, disabled ? false
 
 , meta ? {}
 
@@ -46,6 +49,8 @@
 
 , ... } @ attrs:
 
+assert (!disabled);
+
 # Keep extra attributes from `attrs`, e.g., `patchPhase', etc.
 python.stdenv.mkDerivation (attrs // {
   inherit doCheck;

pkgs/top-level/python-packages.nix

@@ -7,6 +7,7 @@ let
   isPy33 = python.majorVersion == "3.3";
   isPy34 = python.majorVersion == "3.4";
   isPyPy = python.executable == "pypy";
+  isPy3k = strings.substring 0 1 python.majorVersion == "3";
 
   # Unique python version identifier
   pythonName =
@@ -537,6 +538,8 @@ rec {
   avro = buildPythonPackage (rec {
     name = "avro-1.7.6";
 
+    disabled = isPy3k;
+    
     src = fetchurl {
       url = "https://pypi.python.org/packages/source/a/avro/${name}.tar.gz";
       md5 = "7f4893205e5ad69ac86f6b44efb7df72";
@@ -550,7 +553,9 @@ rec {
 
   avro3k = pkgs.lowPrio (buildPythonPackage (rec {
     name = "avro3k-1.7.7-SNAPSHOT";
-
+    
+    disabled = (!isPy3k);
+    
     src = fetchurl {
       url = "https://pypi.python.org/packages/source/a/avro3k/${name}.tar.gz";
       sha256 = "15ahl0irwwj558s964abdxg4vp6iwlabri7klsm2am6q5r0ngsky";