Workaround potential CPython bug.

This works around a potential bug in CPython where the state
associated with a monitoring tool can "leak" in an unexpected
manner.

This patch also removes the use of the functools cache on the
generator for test functions, this to eliminate any potential
state being stored on the code objects associated with the
test functions.

numba/tests/test_sys_monitoring.py

@@ -1,5 +1,4 @@
 import dis
-import functools
 import queue
 import sys
 import threading
@@ -11,7 +10,6 @@
 from numba.core.serialize import _numba_unpickle
 
 
-@functools.cache
 def generate_usecase():
     @jit('int64(int64)',)
     def foo(x):
@@ -285,6 +283,14 @@ def test_disable_from_callback(self):
             self.assertEqual(result, self.call_foo_result)
             callback.assert_called()
         finally:
+            # It is necessary to restart events that have been disabled. The
+            # "disabled" state of the `PY_START` event for the tool
+            # `self.tool_id` "leaks" into subsequent tests. These tests then end
+            # up failing as events that should trigger do not! It's not really
+            # clear why this happens, if it is part of the design or a side
+            # effect of the design, or if this behaviour is simply a bug in
+            # CPython itself.
+            sys.monitoring.restart_events()
             sys.monitoring.register_callback(tool_id, event, None)
             sys.monitoring.set_events(tool_id, NO_EVENTS)
             sys.monitoring.free_tool_id(tool_id)