Fix for Python3.2 compatibility

Current implementation breaks compatibility with Python3.2. This simple fix should work for all Python versions between 2.7 and 3.3 making catkin usable in all these versions.

Unittest works for Python2.7, however it does not work for Python3. I did not adapt the unittest towards Python3 since I guess it is meant for Python2.7 and porting unit tests to Python3 is a whole other topic which should probably be addressed somewhen in the future...