Remove outdated Python versions in CONTRIBUTING
Remove outdated Python versions in CONTRIBUTING

 Refs #2139
getpelican/pelican,zackw/pelican,avaris/pelican,iKevinY/pelican,farseerfc/pelican,zackw/pelican,talha131/pelican,zackw/pelican,ingwinlu/pelican,iKevinY/pelican,ingwinlu/pelican,talha131/pelican,farseerfc/pelican,avaris/pelican,getpelican/pelican