bpo-43700: Replace Zulip badge with Discourse badge (GH-25141)

Automerge-Triggered-By: GH:zware
(cherry picked from commit bef7b26)

Co-authored-by: Erlend Egeberg Aasland <[email protected]>

Author: miss-islington

README.rst

@@ -13,9 +13,9 @@ This is Python version 3.8.8
    :alt: CPython code coverage on Codecov
    :target: https://codecov.io/gh/python/cpython/branch/3.8
 
-.. image:: https://img.shields.io/badge/zulip-join_chat-brightgreen.svg
-   :alt: Python Zulip chat
-   :target: https://python.zulipchat.com
+.. image:: https://img.shields.io/badge/discourse-join_chat-brightgreen.svg
+   :alt: Python Discourse chat
+   :target: https://discuss.python.org/
 
 
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