Marshmallow 4 upgrades

[jdebacker]

This PR addresses Issues #144 and #143 by making ParamTools compatible with Marshmallow 4.0. The Marshmallow guide (which doesn't have a lot of detail) summarizing the changes.

[jdebacker]

So far, this PR has:

• Removed ordered objects (since they are now all ordered by default) (as done in PR Marshmallow 4 upgrades #145)
• Set a context attribute in the BaseValidatorSchema, with the default being an empty dictionary

I'm currently stuck on how to create the fields attribute that is access, e.g., in the Parameters.specification method. Marshmallow had previously inferred fields, but now does not.

cc @hdoupe

[hdoupe]

@jdebacker , opened a pr on this branch with changes that should get this over the finish line if it looks good to you: jdebacker#1

we'll need to make sure to update this before releasing the new paramtools version with the marshmallow 4 changes:
https://github.com/conda-forge/paramtools-feedstock/blob/a33871dc3f5709822de2e99f76b4f4c1d2b0c1cb/recipe/meta.yaml#L23

[martinholmer]

Are the changes in this PR going to be the paramtools 0.20.0 version?
Or are there plans to bump the version number in a different pull request?

[jdebacker]

@hdoupe Thanks for your finishing off this PR. I just bumped the version to 0.20.0 and I think this is ready to go.

Can you give the PR one final review (and merge if you think it's ready)?

[hdoupe]

LGTM, thanks @jdebacker !