Test failures on Python 3.11 due to asyncio.coroutine being removed

[mgorny]

The three following tests are failing on Python 3.11 since it removed deprecated asyncio.coroutine decorator:

============================================================== FAILURES ===============================================================
__________________________________________________ test_as_future_coroutine[asyncio] __________________________________________________

framework = <module 'txaio.aio' from '/tmp/portage/dev-python/txaio-22.2.1/work/txaio-22.2.1/txaio/aio.py'>

    def test_as_future_coroutine(framework):
        '''
        call a coroutine (asyncio)
        '''
        pytest.importorskip('asyncio')
        # can import asyncio on python3.4, but might still be using
        # twisted
        if not txaio.using_asyncio:
            return
    
        errors = []
        results = []
        calls = []
    
>       from asyncio import coroutine
E       ImportError: cannot import name 'coroutine' from 'asyncio' (/usr/lib/python3.11/asyncio/__init__.py)

calls      = []
errors     = []
framework  = <module 'txaio.aio' from '/tmp/portage/dev-python/txaio-22.2.1/work/txaio-22.2.1/txaio/aio.py'>
results    = []

test/test_as_future.py:105: ImportError
______________________________________________ test_explicit_reactor_coroutine[asyncio] _______________________________________________

framework = <module 'txaio.aio' from '/tmp/portage/dev-python/txaio-22.2.1/work/txaio-22.2.1/txaio/aio.py'>

    def test_explicit_reactor_coroutine(framework):
        """
        If we set an event-loop, Futures + Tasks should use it.
        """
        pytest.importorskip('asyncio')
        if txaio.using_twisted:
            pytest.skip()
    
>       from asyncio import coroutine
E       ImportError: cannot import name 'coroutine' from 'asyncio' (/usr/lib/python3.11/asyncio/__init__.py)

framework  = <module 'txaio.aio' from '/tmp/portage/dev-python/txaio-22.2.1/work/txaio-22.2.1/txaio/aio.py'>

test/test_call_later.py:174: ImportError
______________________________________________________ test_is_future_coroutine _______________________________________________________

framework_aio = <module 'txaio.aio' from '/tmp/portage/dev-python/txaio-22.2.1/work/txaio-22.2.1/txaio/aio.py'>

    def test_is_future_coroutine(framework_aio):
        '''
        Returning an immediate value from as_future
        '''
        pytest.importorskip('asyncio')  # 'aio' might be using trollius
>       from asyncio import coroutine
E       ImportError: cannot import name 'coroutine' from 'asyncio' (/usr/lib/python3.11/asyncio/__init__.py)

framework_aio = <module 'txaio.aio' from '/tmp/portage/dev-python/txaio-22.2.1/work/txaio-22.2.1/txaio/aio.py'>

test/test_is_future.py:45: ImportError