6.19.1: circular build dependency in testing procedure

[kloczek]

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.