Bug: Schema class creation should fail if nullable primary key is declared

[DeflateAwning]

        class _HouseSchema(dy.Schema):
            zip_code = dy.String(primary_key=True, nullable=True)
            num_bedrooms = dy.UInt8(nullable=False)
            num_bathrooms = dy.UInt8(nullable=False)
            price = dy.Float64(nullable=False)

Per the new primary key documentation, creating a nullable primary key (e.g., zip_code column in this example) should fail immediately.

I think this would be a AnnotationImplementationError exception. Would be awesome to add a unit test to ensure this fails. I think it should fail during dy.Column() constructor execution.

[AndreasAlbertQC]

Thanks for checking this! In v1, we had not initially forbidden this, and when we introduced this constraint, we did not want to break existing user code. So TLDR we currently throw a warning. We definitely want to make this an exception at some point. Since we've had the warning in for a while now, I'd be open to doing that soonish, or we wait for the other v2 items. @borchero @delsner wdyt?

[borchero]

Let's wait until v2 since we're planning to release this very soon (~next 2 weeks).

[borchero]

Fixed in #173