bpo-31564: NewType can subclass NewType

[topper-123]

It has become possible to create a NewType from an existing NewType, see python/mypy#3465 and python/peps#271.

This is a small update to the documentation as a concequence of the above change.

I've logged this as an issue at #bpo-31564 as per @Mariatta request.

https://bugs.python.org/issue31564

[the-knights-who-say-ni]

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[topper-123]

I've signed the CLA now.

[Mariatta]

This seems good, but I'm not the typing expert..
@ilevkivskyi Maybe you can take another look? Thanks :)

[Mariatta]

No news entry is needed, but I think we should at least have an issue in the bug tracker. @topper-123 can you create an issue, and then add the bpo number to this PR?
Thanks.

[ilevkivskyi]

Thanks! Looks good to me.

[miss-islington]

Thanks @topper-123 for the PR, and @Mariatta for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6.
🐍🍒⛏🤖

[bedevere-bot]

GH-3715 is a backport of this pull request to the 3.6 branch.

[Mariatta]

Thanks for the review @ilevkivskyi.
@topper-123 thanks and congrats on your first contribution to CPython 🌮