[reddit] Add duration to extracted information for posts containing a v.redd.it video

[trevnels]

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• Searched the bugtracker for similar pull requests
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• Read youtube-dl coding conventions and adjusted the code to meet them
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What is the purpose of your pull request?

• Bug fix
• Improvement
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• New feature

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Description of your pull request and other information

Previously, the Reddit extractor would not provide video duration for post videos served through v.redd.it, though this information is provided by the Reddit post API. This change includes the duration key of the API response in the video's extracted information when present. I also added duration to the existing test's info_dict for coverage.

[dstftw]

Read coding conventions.

[trevnels]

@dstftw I believe this is now correct.

[dstftw]

• Checked the code with flake8