Pipeline: Skip except_elements logic if no elements to except

See merge request !688

Chandan Singh authored Aug 21, 2018 and Tristan Van Berkom committed Aug 21, 2018

These new images come with pinned versions of `pytest` and `pylint`:

- `pylint`: `== 2.1.1`
- `pytest`: `>= 3.7`

Chandan Singh authored Aug 21, 2018 and Tristan Van Berkom committed Aug 21, 2018

These packages should only be updated as a deliberate change and the
code should be tested with the new versions to ensure that the test
still pass.

Any future updates should also happen in sync with the
buildstream-docker-images repository so that the testsuite base images
also enforce the same restrictions for the development requirements.

Replaces !684.