Add support for async (generator) fixtures

[malinoff]

https://github.com/pytest-dev/pytest-asyncio has a very convenient feature: async (generator) fixtures. I.e.

@pytest.fixture()
async def connection():
    async with Connection() as conn:
        yield conn

I'd like to also have async (generator) fixtures in pytest-curio.

[geronimo-iia]

Hello, this seem to be the case with the last release