Update libraries.md: add githubpy for python3 client

[michaelliao]

Add a new python3 client for REST API.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
rest/overview/libraries.md	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae	fpt ghec ghes@ 3.7 3.6 3.5 3.4 3.3 ghae

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server
ghae: GitHub AE

[cmwilson21]

@michaelliao Thanks for opening a PR and for your interest in improving the Docs! However, I spoke with the team, and we are not going to accept this addition to the library. 💛

Please take a look at our help wanted section to find open issues you can work on. ✍️

Thanks again for your passion and interest in improving the Docs! 💖