Fix: Replace deprecated dbapi() with import_dbapi() in dialect classes

[leodrivera]

Description

This PR fixes a deprecation warning by replacing the deprecated dbapi() classmethod with import_dbapi() in both Redshift dialect classes. SQLAlchemy has deprecated dbapi() in favor of import_dbapi() to better reflect the method's purpose.

Problem

When using this library with SQLAlchemy, users encounter the following deprecation warning:

SADeprecationWarning: The dbapi() classmethod on dialect classes has been renamed to import_dbapi().
Implement an import_dbapi() classmethod directly on class <class 'sqlalchemy_redshift.dialect.RedshiftDialect_psycopg2'>
to remove this warning; the old .dbapi() classmethod may be maintained for backwards compatibility.

This was reported in: great-expectations/great_expectations#11141

Solution

• Renamed dbapi() to import_dbapi() in Psycopg2RedshiftDialectMixin
• Renamed dbapi() to import_dbapi() in RedshiftDialect_redshift_connector
• Added docstrings to both import_dbapi() methods explaining their purpose

Changes

• sqlalchemy_redshift/dialect.py: Updated both dialect classes to use import_dbapi() instead of dbapi()