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**Description**

I want to specify the cipher to use when creating a SftpHook. Currently this is not possible because there is not a way to propagate the value to CnOpts. 

https://github.com/apache/airflow/blob/master/airflow/providers/sftp/hooks/sftp.py
https://pysftp.readthedocs.io/en/release_0.2.9/pysftp.html?#pysftp.CnOpts
https://pysftp.readthedocs.io/en/release_0.2.9/cookbook.html

**Use case / motivation**

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I want to be able to access SFTP servers that have disabled access from connection with weak ciphers.

tests/providers/sftp/hooks/test_sftp.py
#
#
#
import os
import shutil
import unittest
from unittest import mock
import pysftp
from parameterized import parameterized
from airflow.models import Connection
from airflow.providers.sftp.hooks.sftp import SFTPHook
from airflow.utils.session import provide_session
TMP_PATH = '/tmp'
TMP_DIR_FOR_TESTS = 'tests_sftp_hook_dir'
SUB_DIR = "sub_dir"
TMP_FILE_FOR_TESTS = 'test_file.txt'
SFTP_CONNECTION_USER = "root"
class TestSFTPHook(unittest.TestCase):