Fix GH Actions by removing broken poetry-version-plugin and dropping python 3.6 support

[SimonCW]

Hey @tiangolo, this fixes the GH Action Jobs and, hence, unblocks development. I tried to stick to strictly necessary changes in order to keep this PR small.

Changes:

• Removed dependency on broken poetry-version-plugin
• Updated poetry version in CI Jobs
• Removed support for Python 3.6 (eol + doesn't work with newer poetry)
• Fixed the import of ForwardRef b/c of breaking change in pydantic (Fix imports indirectly from pydantic typing pydantic/pydantic#4358)

I'm setting the __version__ dynamically as outlined here: https://stackoverflow.com/a/67097076/6663432 . I'm not 100% certain that this works as intended everywhere.

[SimonCW]

The version-plugin doesn't work, see tiangolo/poetry-version-plugin#28

[SimonCW]

I think one way forward would be to switch sqlmodel to the hatchling build-backend just as fastapi. The backend would provide the versioning without the broken poetry backend and maintenance effort for @tiangolo would be reduced by using a similar setup in sqlmodel and fastapi.

I might try to migrate sqlmodel to hatchling, but I'm very short on time, atm, and I don't have experience with build-backends.

[github-actions]

📝 Docs preview for commit 068c871 at: https://6477a3b554df5e08a6cc632e--sqlmodel.netlify.app

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📝 Docs preview for commit 3ac39b2 at: https://6477aa124877fa0c3bab4a94--sqlmodel.netlify.app

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📝 Docs preview for commit a72e284 at: https://6477adc78f64150f631c469d--sqlmodel.netlify.app

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📝 Docs preview for commit 06deba8 at: https://6477aff64c30060b290484b9--sqlmodel.netlify.app

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📝 Docs preview for commit a708162 at: https://6477b0c913da53126c3b476a--sqlmodel.netlify.app

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📝 Docs preview for commit 79069b5 at: https://6477b1ec165cac12885ed06b--sqlmodel.netlify.app

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📝 Docs preview for commit 4c3afc7 at: https://6477b67a5c5ca115fc91a4f9--sqlmodel.netlify.app

[peterlandry]

I have some time to throw at this, if there's anything specific that could be done to help it land? Happy to look at a Hatch migration if @tiangolo prefers.

[tiangolo]

Thank you @SimonCW! I wanted to update the poetry-version-plugin first. I might consider moving to Hatch, but for now it's fine. 🤓

As this has several conflicts now and the main problem was solved in another PR, I'll pass on this one, but thanks for the effort! ☕