The `SQLModel` base class declared `__tablename__` both as a `ClassVar`
and as a `@declared_attr` method.  Some type checkers (pyright) see the
descriptor type from `@declared_attr`, so setting
`__tablename__ = "my_table"` in a subclass is rejected as a type
mismatch, even though it works at runtime.

Replace the `@declared_attr` method with a default set in
`SQLModelMetaclass.__new__` via `dict_used`, before class creation.

Fixes fastapi#98.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>