docs: note readline no longer supported in REPL after 3.13

Add a note to the readline module documentation stating that
Python 3.13 and later no longer supports readline in the default REPL,
as per gh-118840. Includes workaround using PYTHON_BASIC_REPL.

Closes gh-137113.

Signed-off-by: Kuang Yu Heng <[email protected]>

Author: Yuheng3107

Doc/library/readline.rst

@@ -7,6 +7,10 @@
 
 .. sectionauthor:: Skip Montanaro <[email protected]>
 
+.. note::
+   The Python REPL after version 3.13 no longer supports readline as per :gh:`118840`.
+   However, readline can still be used if the :envvar:`PYTHON_BASIC_REPL` environment variable is set.
+
 --------------
 
 The :mod:`readline` module defines a number of functions to facilitate