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

Automerge-Triggered-By: GH:zware

Author: Erlend Egeberg Aasland

README.rst

@@ -17,9 +17,9 @@ This is Python version 3.10.0 alpha 6
    :alt: CPython code coverage on Codecov
    :target: https://codecov.io/gh/python/cpython
 
-.. 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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