Deprecate the getopt module in its documentation

The getopt module existed since the initial revision of the Python
source code (1990). The optparse module was added to Python 2.3. When
Python 2.7 added the 3rd argparse module, the optparse module was
deprecated but only in the documentation.

Author: vstinner

Doc/library/getopt.rst

@@ -7,6 +7,10 @@
 
 **Source code:** :source:`Lib/getopt.py`
 
+.. deprecated:: 3.13
+   The :mod:`getopt` module is deprecated and will not be developed further;
+   development will continue with the :mod:`argparse` module.
+
 .. note::
 
    The :mod:`getopt` module is a parser for command line options whose API is