test: use anyio instead of pytest-asyncio

test: use anyio instead of pytest-asyncio

This is in preparation for adding support for other async libraries,
through anyio. AnyIO pytest plugin is used in replacement for
pytest-asyncio:

- either the pytest.mark.asyncio is replaced by pytest.mark.anyio, or,
- we rely on the 'anyio_backend' fixture that is pulled in 'aconn_cls'
  fixture (and hence 'aconn') providing automatic detection for test
  functions using it.

The 'anyio_backend' fixture is parametrized to only use asyncio and
selects the event loop policy we need on Windows platform as previously
done in pytest_sessionstart(), but only for Python version 3.8 or
higher.

This fixture is defined in main conftest.py, as well as in
pool/conftest.py since we'll change the former to support more async
backend while keeping the later asyncio-only for now.

Function test_concurrency_async.py::test_ctrl_c is no longer 'async'
because its code does not directly use asyncio (it's done through a
subprocess); but the 'async def' was needed before in order for
pytest-asyncio to run it since the test module had a global
pytest.mark.asyncio (and we were using the "auto" mode).