[contrib.postgres] copy_from does not accept schema.table notation in most recent psycopg2 versions
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## Description

I'm trying to maintain an old (2018) project that includes a lot of Luigi tasks, amongst which there are some tasks derived from [`CopyToTable`](https://github.com/spotify/luigi/blob/master/luigi/contrib/postgres.py#L299).

In this project, the PostgreSQL database contains several schemas, and some data may be added to tables which are not in default `public` schema through `CopyToTable`-derived tasks.

However `CopyToTable` uses the `cursor.copy_from` method, which has been recently modified in `psycopg2` API (see *e.g* https://github.com/psycopg/psycopg2/issues/1294). Hence using Luigi with a recent `psycopg2` raises an error like `psycopg2.errors.UndefinedTable: relation "schema.table" does not exist`.

## Expected behavior

Taking into account the behavior change in psycopg2, considering `schema.table` notation for Postgres tables that are located in a dedicated schema.

## Minimal Working Example

Let's consider the following Python module (let's call it `luigi_copytotable.py`) :

```python
from luigi.contrib.postgres import CopyToTable
import pandas as pd


class SendToDB(CopyToTable):
    """Insert bike availability data into a PostgreSQL table                                  
    """

    host = "localhost"
    database = "my_db"
    user = "my_username"
    password = "my_password"

    columns = [('a', 'VARCHAR'), ('b', 'INT')]

    @property
    def table(self):
        return 'my_schema.my_table'

    def rows(self):
        df = pd.DataFrame({"a": ["foo", "bar", "wiz"], "b": [1, 2, 3]})
        for idx, row in df.iterrows():
            yield row.values
```

Running `luigi --local-scheduler --module luigi_copytotable SendToDB` throws:

```bash
16:04 $ luigi --local-scheduler --module luigi_copytotable SendToDB_
DEBUG: Checking if SendToDB() is complete
INFO: Informed scheduler that task   SendToDB__99914b932b   has status   PENDING
INFO: Done scheduling tasks
INFO: Running Worker with 1 processes
DEBUG: Asking scheduler for work...
DEBUG: Pending tasks: 1
INFO: [pid 494717] Worker Worker(salt=450412579, workers=1, host=*******, username=my_username, pid=494717) running   SendToDB_()
INFO: Done writing, importing at 2022-09-07 16:04:05.364381
INFO: Creating table my_schema.my_table
ERROR: [pid 494717] Worker Worker(salt=450412579, workers=1, host=*******, username=my_username, pid=494717) failed    SendToDB()
Traceback (most recent call last):
  File "/home/rdelhome/.virtualenvs/jitenv/lib/python3.10/site-packages/luigi/worker.py", line 198, in run
    new_deps = self._run_get_new_deps()
  File "/home/rdelhome/.virtualenvs/jitenv/lib/python3.10/site-packages/luigi/worker.py", line 138, in _run_get_new_deps
    task_gen = self.task.run()
  File "/home/rdelhome/.virtualenvs/jitenv/lib/python3.10/site-packages/luigi/contrib/postgres.py", line 403, in run
    self.copy(cursor, tmp_file)
  File "/home/rdelhome/.virtualenvs/jitenv/lib/python3.10/site-packages/luigi/contrib/postgres.py", line 358, in copy
    cursor.copy_from(
psycopg2.errors.UndefinedTable: relation "my_schema.my_table" does not exist

DEBUG: 1 running tasks, waiting for next task to finish
INFO: Informed scheduler that task   SendToDB__99914b932b   has status   FAILED
DEBUG: Asking scheduler for work...
DEBUG: Done
DEBUG: There are no more tasks to run at this time
DEBUG: There are 1 pending tasks possibly being run by other workers
DEBUG: There are 1 pending tasks unique to this worker
DEBUG: There are 1 pending tasks last scheduled by this worker
INFO: Worker Worker(salt=450412579, workers=1, host=*********, username=my_username, pid=494717) was stopped. Shutting down Keep-Alive thread
INFO: 
===== Luigi Execution Summary =====

Scheduled 1 tasks of which:
* 1 failed:
    - 1 SendToDB()

This progress looks :( because there were failed tasks

===== Luigi Execution Summary =====
```

## Hints for resolution

As suggested in the psycopg2 issue, use `copy_expert` ? Or maybe modify the `if` predicate in https://github.com/spotify/luigi/blob/master/luigi/contrib/postgres.py#L357, to choose the `else` case if `copy_from` is not happy...

(Note: As a temporary solution, I've downgraded my  `psycopg2` version to `<2.9` to make it work.)

## Related issue

See on psycopg2 project: https://github.com/psycopg/psycopg2/issues/1294