DOCS: 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.

[the-knights-who-say-ni]

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[Mariatta]

This PR is against the 3.6 branch. Please make it against the master branch first.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I didn't expect the Spanish Inquisition!. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[topper-123]

Moved to gh-3696 as to merge against master rather than 3.6.

I didn't expect the Spanish Inquisition!

[bedevere-bot]

Nobody expects the Spanish Inquisition!

@Mariatta: please review the changes made to this pull request.