BUG: Getting FileNotFoundError exception if multiple instances of SnmpSession were previously used

[arthur-zzz]

Describe the issue:

Getting a FileNotFoundError exception if multiple instances of SnmpSession were previously used.

Reproduce the code example:

import asyncio
from gufo.snmp import SnmpSession

async def run_multiple_sessions() -> None:
    for i in range(10):
        async with SnmpSession(
            addr="127.0.0.1",
            community="public"
        ) as session:
            async for _ in session.fetch("1.3.6.1.2.1.1"):
                pass

asyncio.run(run_multiple_sessions())

Error message:

Traceback (most recent call last):
  File "/opt/gufo-snmp/run-multiple.py", line 13, in <module>
    asyncio.run(run_multiple_sessions())
  File "/usr/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/opt/gufo-snmp/run-multiple.py", line 10, in run_multiple_sessions
    async for _ in session.fetch("1.3.6.1.2.1.1"):
  File "/opt/gufo-snmp/venv/lib/python3.9/site-packages/gufo/snmp/getbulk.py", line 74, in __anext__
    loop.add_writer(self._fd, w_ev.set)
  File "/usr/lib/python3.9/asyncio/selector_events.py", line 346, in add_writer
    self._add_writer(fd, callback, *args)
  File "/usr/lib/python3.9/asyncio/selector_events.py", line 304, in _add_writer
    self._selector.modify(fd, mask | selectors.EVENT_WRITE,
  File "/usr/lib/python3.9/selectors.py", line 390, in modify
    self._selector.modify(key.fd, selector_events)
FileNotFoundError: [Errno 2] No such file or directory

Python version information

3.9.2 (default, Feb 28 2021, 17:03:44)
[GCC 10.2.1 20210110

Gufo Ping version information

0.1.0

Operation system version

Debian GNU/Linux 11 (bullseye)