No Python 2.7 support for upcoming PCRaster 4.3

PCRaster and Python version 2 have been loyal friends for a long time, but even good things won't last forever. As the official support for Python 2 ends in less than three months, we no longer make efforts in building new versions of PCRaster against Python 2. The upcoming PCRaster 4.3 release (expected later this year) will exclusively support Python 3 on all supported platforms. Modellers really should migrate their model code to Python 3 in case they did not until now.