Update openapi.json

[drphungky]

Updated openapi.json to accept new possible values from YNAB based on their recent changes to account types. Tested these out and they're currently throwing errors.

(If this is...not how to do a pull request forgive me. First one. Relatively new to github and super new to python.)

[dmlerner]

It's a bit more involved to fix this. You'd have to do steps 4 and 5 here: https://github.com/dmlerner/ynab-api/blob/master/regeneration-instructions.md

And if that doesn't break the tests (run with I believe just tox), I could include accept an updated PR. See also the open issue about nullability.