Implement a new plugin manager from scratch to replace Yapsy and add Python 3.12 support (Fix #3719, Fix #3725)

[Kwpolska]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (if applicable).
• I tested my changes.

Description

Yapsy does not work under Python 3.12 due to Python 3.12 carelessly removing things from the standard library (fixes #3719).
To keep Nikola working, a new plugin manager was developed from scratch, and it will be used instead of Yapsy.
This also contains a fix for templates not loading if the plugin places the templates in the directory with __init__.py (fixes #3725).

[felixfontein]

Thanks for working on this! I didn't have much time to review this yet, but I did run some first test and noticed that it doesn't load quite a few of my plugins (Plugin ... does not specify Nikola configuration - it will not be loaded). I'll take a closer look at this probably over the weekend.

[Kwpolska]

I forgot to mention that setting Nikola → PluginCategory in the .plugin file is now mandatory for all plugins. Wrong values will break in unexpected ways (although it might be worth adding a check). I’ll update CHANGES.txt to mention this.

[felixfontein]

With the updated plugin metadata (see also getnikola/plugins#431) everything seems to work well!