[YouTube] Realign `n-sig` processing with yt-dlp #2803

Job	Run time
Linter	16s
Select tests from inputs	1s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 2s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 8s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 4s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 11s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	53s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 36s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 27s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 51s
Run tests (windows-2019, 3.4, cpython, core, bat)	2m 0s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	58s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 7s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	30s
	20m 4s

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