Upgrade for Netbox 2.9

[jeremy-albinet]

Fixes #79 & #64

Swagger from Netbox 2.9 with some changes:

• x-omitempty: true on arrays of NestedTag
• default response on all post/put/patch paths to pretty print errors

I'm available to maintain this 2.9 version.

If you prefer to validate / merge #68, I'm ready to rebase my MR.

Feel free to ask me to squash/rebase/amend my commits.

[awlx]

I think we should merge this @kobayashi. I just merged it in my local branch and it seems to work just fine.