Python 3 Compatibility Updates (step 2), master branch (2019.10.04.)

This is a continuation of !550 (merged), as apparently that didn't do everything correctly just yet.

As @wlampl found, the code was not actually working correctly in its latest version. That was because of the very last update that I added in !550 (merged). 😦 The one that tried to forward the values of PythonInterp_FIND_VERSION and PythonLibs_FIND_VERSION to the find_package(Python...) calls. So I just removed that feature. In the end the standalone builds are not desperately dependent on the 2.7 version specification anyway...

At the same time simplified the LCGConfig.cmake configuration a bit, and made it explicit in the code that we now need CMake 3.12 to use FindPythonInterp.cmake and FindPythonLibs.cmake.

Of course this is still in the service of ATLINFR-3221.