[3.6] bpo-31564: Update typing documentation (GH-3696)

[miss-islington]

Mention that NewType can derive from another NewType.
(cherry picked from commit 039b25d)

https://bugs.python.org/issue31564

[the-knights-who-say-ni]

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[miss-islington]

@topper-123 and @Mariatta: Backport status check is done, and it's a failure ❌ .

[miss-islington]

@topper-123 and @Mariatta: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @Mariatta!