bpo-42128: Add Pattern Matching to What's New

[willingc]

https://bugs.python.org/issue42128

[willingc]

@ramalho If you get a chance, please take a look. Thanks!

[gvanrossum]

Thank you so much Carol! I found a few markup mistakes and have a few more editorial suggestions.

[gvanrossum]

I hit Submit too soon, here are the last three review items. Goodnight!

[Fidget-Spinner]

I haven't read everything in detail, but here's a bunch of suggestions to get the CI checks to pass :).
(mostly just whitespace fixes)

[KunKax]

.

[gvanrossum]

+1 on having Scala *and *Elixir.

[ramalho]

Great quick intro, @willingc thanks! I think the special variable _ deserves more attention. It appears only alone in the last case clause, and then appears again in a discussion of *. Adding an example where _ appears inside a more elaborate pattern without * would clarify the essential meaning of _. Something like ('error', code, _).

I think it would help readers if that example could appear after an example with case _, but before any mention of *.

Also, as @gvanrossum suggested, it's important to have an example without case _ to make it clear that no match means no op.

[gvanrossum]

Thanks! LGTM.

[willingc]

Thanks @gvanrossum, @ramalho, and others who helped review this PR. Your feedback improved the content which should help users with understanding.

[gvanrossum]

You're welcome! @willingc Go ahead and land it.

[bedevere-bot]

@willingc: Please replace # with GH- in the commit message next time. Thanks!