Amends e9f3f8cc64
We switched to find_package(Python3 ...) and that doesn't use
PYTHON_LIBRARY and PYTHON_INCLUDE_DIR. Simply provide the root path for
find_package(Python3 ...).
Change-Id: If862151c095af358be5f35d3d8002d77dcd17aa6
Reviewed-by: Christian Stenger <christian.stenger@qt.io>