TelnetWriter.is_closing() raises AttributeError after TelnetWriter.close() was called.

[jonathanslenders]

Hi @jquast, thanks for telnetlib3!

It looks like calling TelnetWriter.close() sets TelnetWriter._protocol to None.
Because of this, calling TelnetWriter.is_closing() after this raises an AttributeError.

Traceback (most recent call last):
  ...
  File "/home/jonathan/git/project/project/src/project/pty_stream/telnet.py", line 122, in ...
    if not instance.is_closing():
  File "/home/jonathan/git/project/project/src/project/pty_stream/telnet.py", line 161, in is_closing
    result: bool = self.stream_writer.is_closing()
  File "/home/jonathan/.pyenv/versions/3.9.13/lib/python3.9/asyncio/streams.py", line 356, in is_closing
    return self._transport.is_closing()
AttributeError: 'NoneType' object has no attribute 'is_closing'

According to the Python docs, is_closing() should return True after close() was called. It should not raise an AttributeError: https://docs.python.org/3/library/asyncio-stream.html#asyncio.StreamWriter.is_closing

Return True if the stream is closed or in the process of being closed.

What I don't understand is the need here to break circular refs. Asyncio's StreamWriter doesn't do it either. Changing the type of a field from the base class from protocols.BaseProtocol to Optional[protocols.BaseProtocol] is a Liskov violation.

edit:
Similar, .wait_closed() can fail with an AttributeError too:

  ...
  File "/home/jonathan/git/radkit/project/.../telnet.py", line 128, in create_acm
    await instance.stream_writer.wait_closed()
  File "/home/jonathan/.pyenv/versions/3.9.13/lib/python3.9/asyncio/streams.py", line 359, in wait_closed
    await self._protocol._get_close_waiter(self)
AttributeError: 'NoneType' object has no attribute '_get_close_waiter'

[jquast]

I have addressed this in cecceb8, related, #47 that we should not derive from StreamWriter.

The "circular reference " problem is with the undocumented _waiter_closed and _waiter_connected futures, which are used for tests, they are setting future value of "self" to workaround the difficulty of getting a handle on these things in unit tests, I hope to provide basic general purpose or several specific awaitable methods, like "wait_for_naws()", written about here https://github.com/jquast/telnetlib3/blob/master/DESIGN.rst#wait_for_negotiation, so that I can remove those things and special code in close() method entirely.

[jquast]

Closed by release of 2.0.1 https://telnetlib3.readthedocs.io/en/master/history.html#history

Thanks again @jonathanslenders I'm a big fan of your work, may you have a great summer and write some great new codes :)