Only use functions in the limited API (#1545)

This PR removes usage of the only method in rmm's Cython that is not part of the Python limited API. Contributes to rapidsai/build-planning#42

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - https://github.com/jakirkham

Approvers:
  - https://github.com/jakirkham

URL: #1545

python/rmm/rmm/_lib/device_buffer.pyx

@@ -14,7 +14,7 @@
 import numpy as np
 
 cimport cython
-from cpython.bytes cimport PyBytes_AS_STRING, PyBytes_FromStringAndSize
+from cpython.bytes cimport PyBytes_FromStringAndSize
 from libc.stdint cimport uintptr_t
 from libcpp.memory cimport unique_ptr
 from libcpp.utility cimport move
@@ -314,7 +314,7 @@ cdef class DeviceBuffer:
         cdef size_t s = dbp.size()
 
         cdef bytes b = PyBytes_FromStringAndSize(NULL, s)
-        cdef unsigned char* p = <unsigned char*>PyBytes_AS_STRING(b)
+        cdef unsigned char* p = b
         cdef unsigned char[::1] mv = (<unsigned char[:(s + 1):1]>p)[:s]
         self.copy_to_host(mv, stream)