chore: update alembic requirement from >=1.19.0 to >=1.20.0

[dependabot]

Updates the requirements on alembic to permit the latest version.

Release notes

Sourced from alembic's releases.

1.20.0

Released: September 11, 2026

usecase

• [usecase] [batch] Added a warning for the case where an unnamed CHECK constraint on a reflected table is omitted from a batch "recreate" operation. An unnamed CHECK constraint can't be reliably carried over in a batch recreate as it may refer to columns that are being dropped or changed. This omission was previously a silent operation. The presence of any ~sqlalchemy.schema.CheckConstraint in Operations.batch_alter_table.table_args is taken to indicate that the case has been accommodated, and no warning is emitted.

  References: #1846

• [usecase] [autogenerate] Autogenerate now renders a warning comment above any rendered Operations.drop_constraint() directive for which the constraint name is None, as is the case when a constraint that has no name in the model is dropped, most typically within the downgrade() function of a migration that adds an unnamed constraint. A warning is also emitted on the console when the migration script is generated. The directive requires a non-None name in order to be able to emit a "DROP CONSTRAINT" command.

  References: #916

bug

• [bug] [batch] Fixed bug in batch mode where adding a column with a type that generates its own CHECK constraint, such as ~sqlalchemy.types.Boolean or ~sqlalchemy.types.Enum with ~sqlalchemy.types.Boolean.create_constraint set to True, would emit the constraint twice when the table was recreated, once under the name generated by the naming convention in use and once under the name given to the type. The constraint is now emitted once, using the same name that would be used outside of batch mode.

  References: #1768

• [bug] [batch] Fixed bug in batch mode where a CHECK constraint generated by a type such as ~sqlalchemy.types.Boolean or ~sqlalchemy.types.Enum would lose the name established for it by the naming convention in use when the table was recreated, as the constraint was regenerated against the temporary table used for the recreate operation. The naming convention is now resolved against the name of the table being replaced.

... (truncated)

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[dependabot]

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