[3.9] bpo-43087: Fix error in ctypes "Incomplete Types" doc (GH-24404)

The previous "Fundamental data types" section says a c_char_p must be bytes (or None).
(cherry picked from commit 14829b0)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Doc/library/ctypes.rst

@@ -919,9 +919,9 @@ Let's try it. We create two instances of ``cell``, and let them point to each
 other, and finally follow the pointer chain a few times::
 
    >>> c1 = cell()
-   >>> c1.name = "foo"
+   >>> c1.name = b"foo"
    >>> c2 = cell()
-   >>> c2.name = "bar"
+   >>> c2.name = b"bar"
    >>> c1.next = pointer(c2)
    >>> c2.next = pointer(c1)
    >>> p = c1