logging: use item's store for private attributes

This makes things type-safe and properly private.

testing/logging/test_fixture.py

@@ -1,6 +1,7 @@
 import logging
 
 import pytest
+from _pytest.logging import catch_log_handlers_key
 
 logger = logging.getLogger(__name__)
 sublogger = logging.getLogger(__name__ + ".baz")
@@ -136,4 +137,4 @@ def test_caplog_captures_for_all_stages(caplog, logging_during_setup_and_teardow
     assert [x.message for x in caplog.get_records("setup")] == ["a_setup_log"]
 
     # This reaches into private API, don't use this type of thing in real tests!
-    assert set(caplog._item.catch_log_handlers.keys()) == {"setup", "call"}
+    assert set(caplog._item._store[catch_log_handlers_key]) == {"setup", "call"}