Duplicate Python Values

[saulshanabrook]

from @sklam in #393 (comment)

Btw, I notice that cloudpickle does not always make the same byte sequence for the same object if the object pickled by-value. This leads to duplicated egglog PyObject nodes even though the underlying py-object is the same. For example, any class in the main module is not "importable" and will therefore pickled by-value (see cloudpipe/cloudpickle@f5199fe/cloudpickle/cloudpickle.py#L291-L303).