Fix mypy type check error
Fix mypy type check error

Only relevant because we use selftest (which runs mypy) during Docker build, and this error would fail the build.

harrystech/arthur-redshift-etl,harrystech/arthur-redshift-etl,harrystech/arthur-redshift-etl,harrystech/arthur-redshift-etl,harrystech/arthur-redshift-etl