Require CMake 3.15 to ensure policy 0094
Increment minimal CMake version from 3.14 to 3.15.
This ensures that policy CMP0094 is NEW.
This affects the Python_FIND_STRATEGY in CMake module FindPython3.
With the new strategy, we find the main Python minor version, even if some modules are already available with higher minor version number. This resolves a problem discussed in