Log Message:
geography/qgis: Declare python3 incompatible

With 37, the build fails with complaints about future and the wrong
version.  With 27, it builds.  Definitive documentation is scarce, but
signs point to 27 being standard for 2.18, and 3 perhaps working,
perhaps not.  Because qgis 2.18 should be updated to 3.x anyway, don't
worry and just set the variables for now.