test_unittest: skip asynctest for Python 3.11+ (#13777)

asynctest is not supported for Python 3.11+ because `@asyncio.coroutine` was removed in Python 3.11.

Running the test suite with `tox -e py312-asynctest` gives this error:

```
    class _AwaitEvent:
        def __init__(self, mock):
            self._mock = mock
            self._condition = None

>       @asyncio.coroutine
         ^^^^^^^^^^^^^^^^^
E       AttributeError: module 'asyncio' has no attribute 'coroutine'. Did you mean: 'coroutines'?
```

Author: Stu-ops

testing/test_unittest.py

@@ -1322,10 +1322,12 @@ def test_async_support(pytester: Pytester) -> None:
     reprec.assertoutcome(failed=1, passed=2)
 
 
+@pytest.mark.skipif(
+    sys.version_info >= (3, 11), reason="asynctest is not compatible with Python 3.11+"
+)
 def test_asynctest_support(pytester: Pytester) -> None:
     """Check asynctest support (#7110)"""
     pytest.importorskip("asynctest")
-
     pytester.copy_example("unittest/test_unittest_asynctest.py")
     reprec = pytester.inline_run()
     reprec.assertoutcome(failed=1, passed=2)