Render one CHECK for each StrEnum and Literal column

[czpython]

With SQLAlchemy 2.1, druks makemigrations renders the CHECK of a StrEnum or Literal column two more times, as explicit sa.CheckConstraint entries in op.create_table. The column's sa.Enum(..., create_constraint=True) already creates that CHECK. When you apply the revision, each column gets three CHECKs. For a StrEnum, the CHECKs have the same name, so the upgrade fails with DuplicateObject.

Alembic finds a CHECK that a type owns through constraint._create_rule.target. SQLAlchemy 2.1 changed _create_rule to a functools.partial, which has no target. The platform render_item hook now skips each CHECK that has _type_bound set, so only the column type creates the CHECK. A CHECK that a model declares in __table_args__ still renders.

A new test renders the create_table op for a StrEnum column and a Literal column through the hook, and asserts that the output has no CheckConstraint.