pythonPackages.babelgladeextractor: disable for python2

Uses python3 specific calls in setup.py:
```
  File "setup.py", line 23, in _slurp
    with open(path, encoding="utf-8") as fp:
TypeError: 'encoding' is an invalid keyword argument for this function
```

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

@@ -9,6 +9,7 @@
 buildPythonPackage rec {
   pname = "babelgladeextractor";
   version = "0.7.0";
+  disabled = (!isPy3k); # uses python3 specific file io in setup.py
 
   src = fetchPypi {
     pname = "BabelGladeExtractor";