Update new type system discussion links (#16841)

As of the 10th of September 2023 the typing-sig mail list is now
replaced with
[discuss.python.org](https://discuss.python.org/c/typing/32) as
explained
[here](https://mail.python.org/archives/list/typing-sig@python.org/thread/BENDBBUDRCRMTTK2B5HJ7RCKENFD6DW2/)
and [here](https://discuss.python.org/t/about-the-typing-category/34155)

The README.md file was referencing the old link.

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Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

README.md

@@ -41,8 +41,8 @@ To report a bug or request an enhancement:
 
 To discuss a new type system feature:
 
-- discuss at [typing-sig mailing list](https://mail.python.org/archives/list/typing-sig@python.org/)
-- there is also some historical discussion [here](https://github.com/python/typing/issues)
+- discuss at [discuss.python.org](https://discuss.python.org/c/typing/32)
+- there is also some historical discussion at the [typing-sig mailing list](https://mail.python.org/archives/list/typing-sig@python.org/) and the [python/typing repo](https://github.com/python/typing/issues)
 
 What is mypy?
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