qgis-20: large python dependency refactor; forces homebrew python 2.7 for OS < 10.7
qgis-20: large python dependency refactor; forces homebrew python 2.7 for OS < 10.7

- Note: this is in preparation for upcoming major homebrew python dependency changes
  see: Homebrew/homebrew/pull/24842

OSGeo/homebrew-osgeo4mac,OSGeo/homebrew-osgeo4mac,OSGeo/homebrew-osgeo4mac