This turns a patch contributed by @dsu1995 on bpo into a github PR.

The patch ports the logic of set intersection into the intersection of dictviews. It will address performance issues when taking the intersection of large dictviews.

https://bugs.python.org/issue27575

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@rhettinger, this PR is ready for review. I've verified all the code paths are covered by the tests.

Friendly reminder to use "Co-authored by" in the commit message in situations like this, where patch was originally started by someone else. Thanks.

Devguide reference
GitHub article

Hi @Mariatta!

I wasn't sure if that guidance applied here, because I didn't apply @dsu1995 patches and them make a new commit. Instead, I cherry-picked from his git branch.

As such, David Su is already listed as the sole author of his commits.

If this is not the preferred option, I can redo this branch so that it does not have @dsu1995's commits and instead just has a commit where I applied his patch. In that commit I will list him as the Co-Author in the commit message.

Please advise.

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.

@fgregg, please address the code review requests. Also, due to the code review requests, the 'Co-authored by:' seems appropriate.

thanks for finishing this up, @rhettinger! And for the opportunity to contribute to CPython

Thank you @rhettinger! And thank you @fgregg for migrating my bpo patch to a Github PR and cleaning up the code!