[YouTube] Realign `n-sig` processing with yt-dlp #2801

Job	Run time
Select tests from inputs	1s
Linter	17s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 2s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 1s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	59s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 7s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	55s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 21s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 43s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 36s
Run tests (windows-2019, 3.4, cpython, core, bat)	1m 56s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	46s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 12s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	21s
	19m 17s

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