add cython language

[LiamSwayne]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created the topic or collection I'm changing.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

The Cython language is recognized by GitHub's language recognition tool, Linguist, but is not listed in the languages actively developed by GitHub.

[kenyonj]

Thank you for your contribution! Currently we are only accepting additions to collection lists with the new item appended to the end of the list. Please make this change and we can merge it in 😄 .

[ahpook]

Test failures are unrelated to this change, which is now correctly appending the item to the bottom of the list. Merging.