6.19.1: circular build dependency in testing procedure

Looks like `ipykernel` test suite requires `ipyparallel` which requires `ipykernel`.
This creates kind of loop in dependencies when `ipykernel` will be tested in end with installed `ipyparallel` and older version of the `ipykernel`.

For now I don't see any issues in case of `ipykernel` testing however potentially it may be cause some issue however that loop in build dependencies creates some difficulties in case of packaged distribution resources ordering procedures.