[thirtydaysinger] Add Extractor

[kieraneglin]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

Adds extractor for https://www.30daysinger.com.

This site uses Wistia so the the extractor inherits from the existing WistiaIE, but it doesn't share the URL structure expected by WistiaIE or TeachableIE. Even if I modify these extractors to parse 30daysinger URLs, they don't extract the title or description metadata properly.

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)

[kieraneglin]

For viz: I'm not creating a match group for the video index here since that's handled in ThirtyDaySingerBase. Doing it this way allows me to share the logic between the video and playlist extractor