When generating the spec file with the flask spec command, the output should be deterministic. This PR changes the default set class of Schema class to OrderedSet, so that the schema fields will be in order by default.

P.S. There is already a PR in marshmallow for this (marshmallow-code/marshmallow#1896). We can remove this change when that PR is merged and we pin marshmallow >= 4.0.

• fixes Spec output is non-deterministic #373
• fixes Make field order generated by flask spec deterministic #385

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add Version changed or Version added note in any relevant docs and docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue and your username.
• Run pytest and tox, no tests failed.