Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense except for code from yt-dlp where either this or the below has already been asserted.
• 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?

• Bug fix
• Improvement
• New extractor
• New feature

Description of your pull request and other information

This PR revises the processing of the n-sig YT parameter in the YoutubeIE extractor to realign the code with that in yt-dlp.

Some additional methods from the yt-dlp InfoExtractor class have already been back-ported in yt-dlp; this PR adds further such methods needed by the unified code.

Finally, some obvious test failures are fixed, including a new extraction tactic for like_count from likeButtonViewModel in the YT page, as recently implemented in yt-dlp (thanks @Ganesh910, yt-dlp/yt-dlp#8763).