Add PyBytes_FromObject() and PyObject_Bytes()

The comments were taken from the CPython docs.

Closes #2713.

Cython/Includes/cpython/bytes.pxd

@@ -68,6 +68,10 @@ cdef extern from "Python.h":
     # Return value: New reference.
     # Identical to PyBytes_FromFormat() except that it takes exactly two arguments.
 
+    bytes PyBytes_FromObject(object o)
+    # Return value: New reference.
+    # Return the bytes representation of object o that implements the buffer protocol.
+
     Py_ssize_t PyBytes_Size(object string) except -1
     # Return the length of the string in string object string.
 

Cython/Includes/cpython/object.pxd

@@ -181,6 +181,14 @@ cdef extern from "Python.h":
     # equivalent of the Python expression "str(o)". Called by the
     # str() built-in function and by the print statement.
 
+    object PyObject_Bytes(object o)
+    # Return value: New reference.
+    # Compute a bytes representation of object o. Return NULL on
+    # failure and a bytes object on success. This is equivalent to
+    # the Python expression bytes(o), when o is not an integer.
+    # Unlike bytes(o), a TypeError is raised when o is an integer
+    # instead of a zero-initialized bytes object.
+
     object PyObject_Unicode(object o)
     # Return value: New reference.
     # Compute a Unicode string representation of object o. Returns the