Maybe simplify Plugin architecture to get rid of PluginMeta

[sirosen]

I'm considering this after looking into an issue and coming across a comment from JP about this.
Blame shows this as having been in 9ba06f1 .

While once upon a time it might have been possible to hold out hope for unittest2 plugins, I think the time for that has passed.

While such a change could in theory break 3rd party plugins, I think it might be worth it / worth considering. It just means that anyone with a plugin with an __init__ method needs to make sure they call super().__init__.

Some of nose2's own plugins have initializers which don't call the superclass initializer, so imitators may exist.