py: Allow subclass of native object to delegate to the native buffer_p.

Addresses issue #1109.

tests/basics/subclass_native_buffer.py

@@ -0,0 +1,16 @@
+# test when we subclass a type with the buffer protocol
+
+class my_bytes(bytes):
+    pass
+
+b1 = my_bytes([0, 1])
+b2 = my_bytes([2, 3])
+b3 = bytes([4, 5])
+
+# addition will use the buffer protocol on the RHS
+print(b1 + b2)
+print(b1 + b3)
+print(b3 + b1)
+
+# bytearray construction will use the buffer protocol
+print(bytearray(b1))