Use find_package(Python) instead of PythonInterp

[VincentRouvreau]

Modification taken from #151 Fix #139 CMake ~~3.12 is now 5 years old~~ 3.15 is now 4 years old (july 2019), so it should be ok for every one. CMake 3.15 is required for cmake_policy(SET CMP0094 NEW) CMake 3.16 (nov 2019) is required if the user wants to specify a python interpreter (cmake -DPython_EXECUTABLE=/custom/path/to/python3 ...)