🐛 Make generics work with SQLModel as base class

[ryangalamb]

Generics, as documented in pydantic's docs do not work.

Repro: Use the example from pydantic's docs. The typevar type does not get validated.

For example: Response[int](data="not an int") does not raise an error.

Furthermore, json schema generation is broken, as described here: #1002

This PR:

• fixes generics, so they actually validate (instead of silently passing)
• includes thorough tests (based on an example from pydantic's docs)

[ryangalamb]

(I broke the pydantic v1 tests because I added a test and forgot to label it as v2 only. I will fix it when I get back to my computer.)

Fixed. Once #1276 is merged, all this PR's tests should pass.

[svlandeg]

Thanks for the contribution! We're currently trying to catch up with the backlog of PRs, and will get back to you once we've had time to review this 🙏