bpo-34427: islice values when calling MutableSequence.extend on self

[KittenHero]

https://bugs.python.org/issue34427

[the-knights-who-say-ni]

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[rhettinger]

• We don't want to create another dependency.

• Instead, is it better to use plain Python with no external calls.

• Instead of an islice() approach, emulate what was done for list_extend() in Objects/listobject.c and for deque_extend() in Modules/_collectionsmodule.c. When "values is self", create a copy first using "list(values)".

• The patch also needs tests and a news blurb.

[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 have made the requested changes; please review again. 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.

[KittenHero]

Thanks, I will have a look at those

[KittenHero]

I made the requested changes I think