fix(luigi/contrib/postgres.py): replace copy_from with copy_expert

[delhomer]

Description

copy_from does not accept 'schema.table' notation since psycopg2.9. Then a bug arises in Luigi if the table refers a schema name.

This PR modifies the copy method behavior, by refering copy_expert instead of copy_from, as suggested in psycopg/psycopg2#1294.

Motivation and Context

I'm trying to insert some data into a table that is stored into a specific schema (which is not the public schema), and remain stucked at this stage. See #3198 for details.

Have you tested this? If so, how?

Design the following task class and run luigi --local-scheduler --module fulltablename InsertIntoDB :

# fulltablename.py
class InsertIntoDB(CopyToTable):

    table = "dummy_schema.dummy_table"
    ... # other mandatory CopyToTable attributes

Fix #3198