[YouTube] Rework alert handling, etc

[dirkf]

Boilerplate: own+yt-dlp code, bug fix+improvement

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Description of your pull request and other information

• Reworks alert handling: fixes Crash in dev mode when trying to download private YouTube-playlist #33196
• Corrects some latent typos from [YouTube, misc] Back-ports from yt-dlp for broken YT player 2b83d2e0 and later #33189
• Modernises and simplifies some traversals.

As usual, (presumably) undependable dependencies caused the previously functional commit workflow to break, so now wheel (whose installation from Python3.4's pip-19.1 began to fail) and setuptools are always omitted when installing pip.