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>
