Replace task_state with tasks in dask test

[mrocklin]

This internal state was changed in the latest release

• Closes Broken distributed test #1903 (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[fmaussion]

Thanks @mrocklin ! Could you change the version number checking in the test decorator as well? (@pytest.mark.skipif(distributed.__version__ <= '1.19.3' should become <1.20.2)

[mrocklin]

Why this version number in particular?

[mrocklin]

I believe that this change will be valid across a number of versions, including the most recent.

[fmaussion]

Why this version number in particular?

Sorry, I thought the namespace change happened in v1.20.2 . If the tasks attribute was available in earlier versions of distributed we can keep the decorator as it is.