Feature request: allow specifying a list of values in `context`

[mgorny]

I would like to request the ability to specify a list values in context rather than just a scalar. This is a bit related to #971, but that bug seems more focused on the Jinja expression being flattened, while I'm interested in ability to use YAML lists.

For example, torchvision-feedstock originally skipped tests like this:

    # skip test_url_is_accessible instead of hitting 20+ servers per run, since
    # each server might be occasionally unresponsive and end up failing our CI
    {% set tests_to_skip = "test_url_is_accessible" %}
    # spurious failure because upstream skip (Image.__version__ >= "7") does not trigger for Pillow "10"
    {% set tests_to_skip = tests_to_skip + " or (test_transforms and test_adjust_saturation)" %}
    # osx warns with nnpack if there is no AVX2, see conda-forge/pytorch-cpu-feedstock#56
    {% set tests_to_skip = tests_to_skip + " or test_adjust_sharpness" %}  # [osx]

which is quite cumbersome, but permits including comments.

What I'd ideally like to be able it to specify:

context:
  tests_to_skip:
    # skip test_url_is_accessible instead of hitting 20+ servers per run, since
    # each server might be occasionally unresponsive and end up failing our CI
    - test_url_is_accessible
    # spurious failure because upstream skip (Image.__version__ >= "7") does not trigger for Pillow "10"
    - (test_transforms and test_adjust_saturation)
    # osx warns with nnpack if there is no AVX2, see conda-forge/pytorch-cpu-feedstock#56
    - if: osx
      then: test_adjust_sharpness

that would create a Jinja list that I could afterwards combine via ${{ tests_to_skip | join(" or ") }} or likewise.

I suppose getting conditions in there may be problematic, but even without that part, I think the feature would be really helpful.

For contrast, right now I'm working around the problem via:

context:
  tests_to_skip: >
    ${{ 'skip test_url_is_accessible instead of hitting 20+ servers per run, since' if 0 }}
    ${{ 'each server might be occasionally unresponsive and end up failing our CI' if 0 }}
    test_url_is_accessible
    ${{ 'spurious failure because upstream skip (Image.__version__ >= "7") does not trigger for Pillow "10"' if 0 }}
    or (test_transforms and test_adjust_saturation)
    ${{ 'osx warns with nnpack if there is no AVX2, see conda-forge/pytorch-cpu-feedstock#56' if 0 }}
    ${{ "or test_adjust_sharpness" if osx }}

and while it serves the primary purpose (having comments in place), it is kinda ugly.