Two problems with memoryview:
- The buffer interface of memoryview leaks a reference:
    str(memoryview(b'text'), 'utf-8')

- memoryview does not implement tp_traverse and tp_clear, so reference
cycle cannot be collected, as with:

import gc
class MyBuf(bytes): pass

def f():
    buf = MyBuf(b'abc')
    m = memoryview(buf)
    buf.m = m
    gc.collect();gc.collect();gc.collect()

each call to f() leaks 6 references.