Review 3rd party plugin-engine implementations in replacement of byexample's one

[eldipa]

Describe the feature you'd like
So far byexample uses a home-made plugin engine. In byexample 10.5.0 we had to change it to use the new import-mechanism of Python 3.10 and it wasn't the smoothest experience.

In #220 we found that modules loaded in this way are not pickle-able by default and they are not compatible with the way that multiprocessing spawn sub processes.

Moreover the current plugin-engine has a poor support plugin versioning, conditional loading, and loading order (for dependencies).

This ticket if for review which alternatives exist.

[eldipa]

Some implementations to review: https://github.com/mitsuhiko/pluginbase, https://github.com/Rockhopper-Technologies/pluginlib, http://yapsy.sourceforge.net/.

This post has some other suggestions too: https://wehart.blogspot.com/2009/01/python-plugin-frameworks.html