Remove marshmallow from result/ and requirements

[mtreinish]

Summary

This commit finishes the process of removing marshmallow from terra. It
rebuilds the result class as bare python classes or SimpleNamespace
subclasses (for classes that allow arbitrary fields). After the result
class has been converted there is nothing using marshmallow or all the
support code in qiskit/validation. So this continues the removal process
by removing those and removing marshmallow and marshmallow-polyfield
from the requirements list.

Details and comments

This commit depends on #4027 and #4016 and can't be merged (or tested) without
those applied.

[mtreinish]

This is still on hold because this will break the ibmq-provider until Qiskit/qiskit-ibmq-provider#553 is fixed. Once that's done we can safely merge this. (this is also what's blocking the docs builds because the ibmq provider fails to import with this applied)

[jyu00]

A couple minor comments but overall looks good!

[jyu00]

LGTM!

[taalexander]

This looks fine. The only question I have is do we make it clear that to_dict and from_dict are designed to serialize/deserialize JSON? It might be possible to the wrong idea from the function name alone.

[mtreinish]

Honestly, I'm not sure if that's a concern for this PR. Those methods are there for backwards compatibility with marshmallow only, if I could I would have just removed them. Ideally if I were to add json support to these classes I would have done it with to_json/from_json (or maybe dump_json) or probably even better would just be save and load and had it actually do the serialization. In practice though I don't know if people (outside of the ibmq provider) are actually using it with json, the places I would expect it to be used don't (like ignis for saving experimental results relies on pickles and ignores them). Other (non-ibmq) providers typically actually do from_dict but not from loading json instead from a manually constructed dict, like:

• https://github.com/Qiskit/qiskit-honeywell-provider/blob/a08a4d12bb0493a9c40462cba1b4e23c4ea2972d/qiskit/providers/honeywell/honeywelljob.py#L303-L311
• https://github.com/Qiskit/qiskit-aqt-provider/blob/master/qiskit/providers/aqt/aqt_job.py#L90-L106
• https://github.com/Qiskit/qiskit-terra/blob/master/qiskit/providers/basicaer/qasm_simulator.py#L417-L427

mostly because the marshmallow based classes had poor documentation and if you were going to construct a Results object using the constructors it relied on three layers of nested class construction so from_dictwas much easier.

[jyu00]

The to_dict and from_dict don't really do JSON (de)serialization, as they don't convert non-JSON-serializable objects (e.g. complex) to JSON serializable. They really just convert an object to its dictionary representation.