Move select, selectors, asyncio to section 18 (IPC).

Doc/library/concurrency.rst

@@ -20,9 +20,6 @@ multitasking). Here's an overview:
    subprocess.rst
    sched.rst
    queue.rst
-   select.rst
-   selectors.rst
-   asyncio.rst
 
 
 The following are support modules for some of the above services:

Doc/library/ipc.rst

@@ -18,6 +18,9 @@ The list of modules described in this chapter is:
 
    socket.rst
    ssl.rst
+   select.rst
+   selectors.rst
+   asyncio.rst
    asyncore.rst
    asynchat.rst
    signal.rst