docs/library: Add CPython docs xref to each pertinent module.

Cross-reference text/link is implemented as RST substitution, so easy to
consistently.

docs/library/uio.rst

@@ -4,6 +4,8 @@
 .. module:: uio
    :synopsis: input/output streams
 
+|see_cpython_module| :mod:`python:io`.
+
 This module contains additional types of stream (file-like) objects
 and helper functions.