Unfortunately, Leap has official Python API only for Python2 (this is what is bundled with SlicerLeapMotion extension).
Slicer-4.11 uses Python3, so you need to build build and wrap the LeapMotion API yourself. It is not very simple (see some attempts described here), but should be possible, since Slicer uses standard Python 3.6.7, so if you can make or find anything that works with this Python version then it will work in Slicer, too.