feat(ftp): create a FTP client to keep conn alive

[luabida]

[2023-03-30 08:23:38,158] {{dagbag.py:341}} ERROR - Failed to import: /opt/airflow/dags/brasil/sinan.py
Traceback (most recent call last):
  File "/opt/conda/envs/epigraphhub/lib/python3.9/site-packages/airflow/models/dagbag.py", line 337, in parse
    loader.exec_module(new_module)
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/opt/airflow/dags/brasil/sinan.py", line 429, in <module>
    task_flow_for(disease)
  File "/opt/airflow/dags/brasil/sinan.py", line 92, in task_flow_for
    prelim_years = list(map(int, dis.get_years('prelim')))
  File "/opt/conda/envs/epigraphhub/lib/python3.9/site-packages/pysus/online_data/__init__.py", line 685, in get_years
    finais_paths, prelim_paths = self._finals_prelims_dbcs()
  File "/opt/conda/envs/epigraphhub/lib/python3.9/site-packages/pysus/online_data/__init__.py", line 718, in _finals_prelims_dbcs
    prelims = self.FTP.nlst(f"{DB_PATHS['SINAN'][1]}/{code}BR*.dbc")
  File "/opt/conda/envs/epigraphhub/lib/python3.9/ftplib.py", line 553, in nlst
    self.retrlines(cmd, files.append)
  File "/opt/conda/envs/epigraphhub/lib/python3.9/ftplib.py", line 462, in retrlines
    with self.transfercmd(cmd) as conn, \
  File "/opt/conda/envs/epigraphhub/lib/python3.9/ftplib.py", line 393, in transfercmd
    return self.ntransfercmd(cmd, rest)[0]
  File "/opt/conda/envs/epigraphhub/lib/python3.9/ftplib.py", line 353, in ntransfercmd
    host, port = self.makepasv()
  File "/opt/conda/envs/epigraphhub/lib/python3.9/ftplib.py", line 327, in makepasv
    untrusted_host, port = parse227(self.sendcmd('PASV'))
  File "/opt/conda/envs/epigraphhub/lib/python3.9/ftplib.py", line 839, in parse227
    raise error_reply(resp)
ftplib.error_reply: 200 NOOP command successful.