- added MaybeSet generic type to eliminate boilerplate typing.

The add_user() example in examples/aio_pika_server.py is not yet used
by the example client, so it can be fixed for mypy using dataclasses
similar to the example add_user() with dataclasses at the end of README.rst.

Issue #91 shows an AssertionError when a connected aio_pika client
which didn't need to open a result queue calls client.close().

It should be possible to call client.close() also in this case
to close the connection to the RabbitMQ server, because when
either the client was interrupted, had nothing to do before
closing, it, this is the normal state.

Also replace the other assertions in the aio_pika client's close()
with simple checks which don't abort the function to close any
other remaining connection elements or futures which are not
properly handled when simply aborting with an assertion.

For details and a test case, see issue #91.

Fix AssertionErrors in close() of aio_pika client.

examples/aio_pika_server.py: Use asyncio.new_event_loop() for 3.10+ & mypy fixes

examples/aio_pika_server.py: Fix example add_user() to not trigger mypy

- refactoring done
- dependencies updated
- python 3.11 support added