rebulk support for python 2.7 dropped in rebulk 3.0.0

[gene1wood]

In rebulk 3.0.0 which was released December 2020, support for python2.7 was dropped.

One effect is that because Travis is configured to build on python 2.7, all PRs since that point are failing their travis tests

You could either drop support for python 2.7 in subliminal and update the .travis.yml or limit rebulk to less than version 3