make bytes(o) respect __bytes__ #2415

This adds two new C-API functions: PyObject_Bytes and PyBytes_FromObject.

Reviewer: Barry
diff --git a/Doc/c-api/bytes.rst b/Doc/c-api/bytes.rst
index 916e20e..c9a114a 100644
--- a/Doc/c-api/bytes.rst
+++ b/Doc/c-api/bytes.rst
@@ -118,6 +118,12 @@
    arguments.
 
 
+.. cfunction:: PyObject* PyBytes_FromObject(PyObject *o)
+
+   Return the bytes representation of object *o* that implements the buffer
+   protocol.
+
+
 .. cfunction:: Py_ssize_t PyBytes_Size(PyObject *o)
 
    Return the length of the bytes in bytes object *o*.