Use DeclarativeBase as base class for SA models

[jdavcs]

Post-SA2.0 model fixes (cherry-picked from #17662):

1. Use proper DeclarativeBase as base class, as per sqlalchemy documentation.

2. mypy bug workaround no longer needed as we are no longer specifying a metaclass.

3. Use FromClause as the correct type instead of Table

4. Fix table/__table__/HasTable for models. HasTable was a hack to accommodate declarative mapping without changing thousands of lines that referred to the table attribute of model instances.

   The one type:ignore added to managers.datasets can be removed after we map DatasetInstance models declaratively.

How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

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[jdavcs]

Test failure unrelated.

[mvdbeek]

DatasetInstance doesn't look right here. If you do need to be more precise consider parameterizing this with a generic ?

[jdavcs]

We need a union here only because hda and ldda are not mapped declaratively. model_class is the type for all SA models. All models but these two are mapped declaratively, which gives them a metadata instance variable. The hda and ldda models also have a metadata variable, which is why they can't be mapped declaratively (they will be eventually). Now, all declaratively mapped models inherit from DeclarativeBase. So the closest way to describe the type of all our models is the union of DeclarativeBase and the 2 non-declaratively mapped models - which both subclass DatasetInstance. This is tighter than the previous implementation (_HasTable, before that _BaseModel) which relied on the parent having a table variable.

[jdavcs]

Already merged as part of #17662