Revert "[Serialization] Update CloudPickle to 1.6.0 (#9694)" (#10460)

This reverts commit f0c3910d59.

python/ray/includes/serialization.pxi

@@ -1,5 +1,6 @@
 from libc.string cimport memcpy
 from libc.stdint cimport uintptr_t, uint64_t, INT32_MAX
+from libcpp cimport nullptr
 import cython
 
 DEF MEMCOPY_THREADS = 6
@@ -115,9 +116,6 @@ cdef class SubBuffer:
             <const char*> self.buf, self.len)
 
     def __getbuffer__(self, Py_buffer* buffer, int flags):
-        if flags & cpython.PyBUF_WRITABLE:
-            # Ray ensures all buffers are immutable.
-            raise BufferError
         buffer.readonly = self.readonly
         buffer.buf = self.buf
         buffer.format = <char *>self._format.c_str()