CentOS 7 will go EOL later this year, and infra wants to drop the nodes
soon-ish -- don't make them wait on our account.
The only major loss is py2 probe tests, but officially, yoga was the
last release we pledged to support py2.
Change-Id: I8f6c247c21f16aa4717569cc69308f846c6a0245
bd3b2256a9