ReadOnlyDict/List pickle compatibility and serialization tests

[shwang]

Makes ReadOnly{Dict,List} picklable. Also adds tests for pickle, json serialization.

The main changes to the read-only custom types were adding a __reduce__ method and simplifying the various __init__ calls a bit by using using explicit class.__init__ calls rather than super().__init__ (h/t @Qwlouse #508 (comment) ).

Closes #499. Closes #738.

[shwang]

CI is outputting ##[warning]An image label with the label macos-10.13 does not exist. on cancelled jobs

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[thequilo]

And an item open for discussion that I already mentioned in #770: Should we make the pickled type a read-only container or list and dict?

[thequilo]

The configuration for checks on OSX was updated on the master branch. They should pass after you merge the master into your PR.

[boeddeker]

And an item open for discussion that I already mentioned in #770: Should we make the pickled type a read-only container or list and dict?

As far as I know, are the read only types introduced to prevent accidental modifications of the config
and they should act like the build in types.
So

config = deepcopy(config)
config[...] = ...

should work, because it is obvious that the user took care to not change the global config.
If can only work, then the pickled type is list and dict

[thequilo]

As far as I know, are the read only types introduced to prevent accidental modifications of the config
and they should act like the build in types.

Yes, that's right. The question is if we want to allow

config = pickle.loads(pickle.dumps(config))
config[...] = ...

In most cases that I can imagine, we want to lose the read-only properties when pickling (e.g., when saving to disk), but in some scenarios (e.g., multiprocessing, ray-tune) it might be safer to keep the read-only status.
In #663 it was suggested to make the experiment pickleable, which would enable passing it between processes. In that case, we definitely want to keep things read-only.

[shwang]

Having ReadOnlyDict pickled as dict seems unexpectedly asymmetrical to me.

As @thequilo alluded to in this comment, I think it's better to have the user make a copy first to dict if they want to explicitly save as dict. We already have to do the same thing if they want to make modifications before pickling.

[thequilo]

@shwang Thank you!