[Misc] Under-the-hood fixes, improvements and tests (pt 3)

[dirkf]

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

## 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

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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:

• Except for code from yt-dlp, for which this or the below have been asserted, 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?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

After merging #32481, one further change was needed. Meanwhile new flake8 Linter rules appeared. This PR addresses both.

To pass new Linter rules, direct comparison to types is reworked and at the same time isinstance() tests (especially for str/unicode/bytes) are revised, avoiding actual Python version tests as far as possible