Fix-up test_pickle_empty

[jakirkham]

Builds on and extends PR ( #4595 ). Should address a CI failure seen in PR ( cloudpipe/cloudpickle#432 ).

Switches to using the internal utility class MemoryviewHolder instead of NumPy to allow testing without NumPy. Calls serialize directly with pickle as the only option (instead of calling pickle_dumps). Though also asserts pickling did occur. Also includes some other minor fixes.

Only tries manipulating writability of frames with pickle protocol 5 in use. This doesn't work for earlier pickle protocols as there is only one bytes object containing everything (and no way to inspect what went into it, let alone reproduce it).

cc @madsbk @pierreglaser

• Closes #xxxx
• Tests added / passed
• Passes black distributed / flake8 distributed / isort distributed

[jakirkham]

@dask/maintenance could someone please review? 🙂

This is needed to fix cloudpickle's CI, which runs these tests.

[jakirkham]

Planning on merging EOD tomorrow if no comments

[jakirkham]

Thanks Jacob! 😄

[jakirkham]

To help catch the kind of issue seen in cloudpickle's CI earlier, am adding explicit testing of pickle protocol 4 & 5 in PR ( #5313 ).