1.4.1: pytest warnings

[kloczek]

Looks like latest pytest shows some warnings

============================================================================= warnings summary =============================================================================
tests/test_asynctestcase.py:41
  /home/tkloczko/rpmbuild/BUILD/aiounittest-1.4.1/tests/test_asynctestcase.py:41: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
    def test_yield_async_add(self):

tests/test_asynctestcase.py:56
  /home/tkloczko/rpmbuild/BUILD/aiounittest-1.4.1/tests/test_asynctestcase.py:56: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
    def test_yield_async_add(self):

tests/test_asynctestcase_get_event_loop.py:38
  /home/tkloczko/rpmbuild/BUILD/aiounittest-1.4.1/tests/test_asynctestcase_get_event_loop.py:38: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead
    def test_yield_async_add(self):

-- Docs: https://docs.pytest.org/en/stable/warnings.html