Closes #18959: move optparse and imp to new "superseded modules" chapter

Doc/library/allos.rst

@@ -16,7 +16,6 @@ but they are available on most other systems as well.  Here's an overview:
    io.rst
    time.rst
    argparse.rst
-   optparse.rst
    getopt.rst
    logging.rst
    logging.config.rst

Doc/library/index.rst

@@ -73,4 +73,5 @@ the `Python Package Index <http://pypi.python.org/pypi>`_.
    misc.rst
    windows.rst
    unix.rst
+   superseded.rst
    undoc.rst

Doc/library/modules.rst

@@ -12,7 +12,6 @@ The full list of modules described in this chapter is:
 
 .. toctree::
 
-   imp.rst
    zipimport.rst
    pkgutil.rst
    modulefinder.rst

Doc/library/superseded.rst

@@ -0,0 +1,14 @@
+.. _superseded:
+
+******************
+Superseded Modules
+******************
+
+The modules described in this chapter are deprecated and only kept for
+backwards compatibility. They have been superseded by other modules.
+
+
+.. toctree::
+
+   optparse.rst
+   imp.rst