Add notice of Python 3.9 end of support to README.rst (boto#3589)

---------

Co-authored-by: jonathan343 <43360731+jonathan343@users.noreply.github.com>

Author: nateprewitt

README.rst

@@ -13,9 +13,14 @@ Botocore is maintained and published by `Amazon Web Services`_.
 Notices
 -------
 
+On 2026-04-29, support for Python 3.9 will end for Botocore. This follows the
+Python Software Foundation `end of support <https://peps.python.org/pep-0596/#lifespan>`__
+for the runtime which occurred on 2025-10-31.
+
 On 2025-04-22, support for Python 3.8 ended for Botocore. This follows the
 Python Software Foundation `end of support <https://peps.python.org/pep-0569/#lifespan>`__
 for the runtime which occurred on 2024-10-07.
+
 For more information, see this `blog post <https://aws.amazon.com/blogs/developer/python-support-policy-updates-for-aws-sdks-and-tools/>`__.
 
 .. _`Amazon Web Services`: https://aws.amazon.com/what-is-aws/