bpo-46771: Remove two controversial lines from Task.cancel() (GH-31623)

Also from the _asyncio C accelerator module,
and adjust one test that the change caused to fail.

For more discussion see the discussion starting here:
https://github.com/python/cpython/pull/31394#issuecomment-1053545331

(Basically, @asvetlov proposed to return False from cancel()
when there is already a pending cancellation, and I went along,
even though it wasn't necessary for the task group implementation,
and @agronholm has come up with a counterexample that fails
because of this change.  So now I'm changing it back to the old
semantics (but still bumping the counter) until we can have a
proper discussion about this.)

Lib/test/test_asyncio/test_tasks.py

@@ -514,7 +514,11 @@ async def task():
             self.assertTrue(t.cancel())
             self.assertTrue(t.cancelling())
             self.assertIn(" cancelling ", repr(t))
-            self.assertFalse(t.cancel())
+
+            # Since we commented out two lines from Task.cancel(),
+            # this t.cancel() call now returns True.
+            # self.assertFalse(t.cancel())
+            self.assertTrue(t.cancel())
 
             with self.assertRaises(asyncio.CancelledError):
                 loop.run_until_complete(t)