Migration of machines to gitlab-ci

The buildbots have been languishing and not been doing much useful work, so I’d like to propose dropping the remaining three builds:

  • taanab: python2
  • duma: osmesa/ospray
  • adora: cuda (see this MR and discussion for CUDA progress)

for task items in this issue so that we can have Do: test talk to gitlab-ci reliably.

Any objections?