ENH: Fixes for python 3 support

- The has_key method does no more exist for python3, it was just replaced by a normal "if 'WRAPITK_PYTHON_PATH' in" check.
- execfile() is deprecated in python 3, it was replaced by the newer exec() version (only for python 3)

Change-Id: I82773cf0ecd1ef68341f8c9c9460031bed372c6e
Merge topic 'Python3'

a5a09a9f ENH: Fixes for python 3 support