#9604 breaks cloning in pipelines. At least the "strict" cloning I wanted to enforce for 0.20.
The patch in #9604 results in a deprecation warning each time a pipeline is cloned because __init__ modifies steps.

I'm not entirely sure what to do about that, but if we want the deprecation of clone to go through (which I want), we need to refactor pipeline to not modify steps - which is probably hard to do in a backward compatible manner.

Another option would be to not copy a lists if it's passed in, and deprecate the behavior of passing a tuple. How did this work in 0.18?