Fix Python 3.6 StreamWriter compatibility issue

Fix Python 3.6 StreamWriter compatibility issue

The StreamWriter.wait_closed() method was introduced in Python 3.7, so
attempting to use it with Python 3.6 raises an exception.  This has not
been noticed before because awaiting StreamWriter.wait_closed() is the
last action taken for each TCP connection and unhandled exceptions were
not causing the scripts based on AsyncServer to exit prematurely until
the previous commit.

As per Python documentation [1], awaiting StreamWriter.wait_closed()
after calling StreamWriter.close() is recommended, but not mandatory, so
try to use it if it is available, without taking any fallback action in
case it isn't.

[1] https://docs.python.org/3.13/library/asyncio-stream.html#asyncio.StreamWriter.close

(cherry picked from commit 715bd1b6678ae591b6edf34c1d5d748f04ad22d0)