bpo-43571: Add IPPROTO_MPTCP macro (GH-24946)

Add IPPROTO_MPTCP constant when defined in the system headers.
diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
index 2255b82..31d804c 100755
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -610,6 +610,9 @@
    .. versionchanged:: 3.9
        The CAN_J1939 protocol was added.
 
+   .. versionchanged:: 3.10
+       The IPPROTO_MPTCP protocol was added.
+
 .. function:: socketpair([family[, type[, proto]]])
 
    Build a pair of connected socket objects using the given address family, socket