Replace asynctest with unittest.IsolatedAsyncioTestCase

[rsekman]

asynctest is abandoned and doesn't work on Python 3.11 and newer, where it fails to import because asyncio.coroutine is gone. That made the whole test suite uncollectable.

Everything asynctest provided is in the standard library now:

  asynctest.TestCase        -> unittest.IsolatedAsyncioTestCase
  asynctest.CoroutineMock   -> unittest.mock.AsyncMock
  asynctest.patch           -> unittest.mock.patch
  async def setUp()         -> async def asyncSetUp()

The exception is ClockedTestCase, which is now in tests/testutils.py. Like asynctest's, its clock starts at 0 and only moves when advance() says so, so tests never wait for real time to pass. advance() steps to each scheduled callback in turn instead of jumping straight to the target time, because a callback that reschedules itself must run once per interval, not once in total.

test_TUI_completion_candidates_for_posargs was skipped because of an asynctest bug and passes now, so it's no longer skipped.

The minimum Python version is now 3.10.