[3.12] gh-77894: Fix a crash when the GC breaks a loop containing a memoryview (GH-123898) (GH-123937)

Now a memoryview object can only be cleared if there are no buffers
that refer it.
(cherry picked from commit a1dbf2ea69)

Lib/test/test_memoryview.py

@@ -18,6 +18,10 @@ import struct
 from test.support import import_helper
 
 
+class MyObject:
+    pass
+
+
 class AbstractMemoryTests:
     source_bytes = b"abcdef"
 
@@ -228,8 +232,6 @@ class AbstractMemoryTests:
                     self.m = memoryview(base)
             class MySource(tp):
                 pass
-            class MyObject:
-                pass
 
             # Create a reference cycle through a memoryview object.
             # This exercises mbuf_clear().
@@ -656,5 +658,26 @@ class OtherTest(unittest.TestCase):
             m[0] = MyBool()
         self.assertEqual(ba[:8], b'\0'*8)
 
+    def test_buffer_reference_loop(self):
+        m = memoryview(b'abc').__buffer__(0)
+        o = MyObject()
+        o.m = m
+        o.o = o
+        wr = weakref.ref(o)
+        del m, o
+        gc.collect()
+        self.assertIsNone(wr())
+
+    def test_picklebuffer_reference_loop(self):
+        pb = pickle.PickleBuffer(memoryview(b'abc'))
+        o = MyObject()
+        o.pb = pb
+        o.o = o
+        wr = weakref.ref(o)
+        del pb, o
+        gc.collect()
+        self.assertIsNone(wr())
+
+
 if __name__ == "__main__":
     unittest.main()