This would address the point raised in https://github.com/apache/arrow/pull/742 by [~wesmckinn] and replace the current C Python API with Cython.
- More uniform with the rest of Arrow (especially pyarrow)
- can share code with pyarrow
- makes error handling more uniform
- easier to program than the C Python API
As part of this, we would also fix the visibility of symbols in the plasma library and port the python tests we have for plasma (which are currently in the Ray repo) to this repo.