Expose effective class construction properties

[hynek]

ref #602

This is for now just private and the impact zone is limited. But I'm confident it can help us to simplify a bunch of code.

I'm especially rather unhappy with the weird entanglement of _ClassBuilder that doesn't really have a clear purpose (anymore?).

But let's talk in general first!

[hynek]

@Tinche what did we agree to call the inspect function that extracts the props?

[Tinche]

LGTM. But where's inspect? :D

[hynek]

Unfortunately, I'm apparently too dumb to make this work with Mypy. So unless someone can help here, I'll just make it private and ship it. The refactoring are worth it on their own.

[hynek]

Some more hours wasted later, I got it working by using our internal stuff to create classes and not using enums in the type hints.

It seems like any type of type hint triggers Mypy to look at our code which ends in disaster. This also means there's probably no gradual way to make the code base fully typed. :(

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please final review and merge @Tinche – 25.4.0 will follow ASAP

please also look at https://attrs--1454.org.readthedocs.build/en/1454/api.html#attrs.inspect

[hynek]

(I have changed some of the field names to make them more idiomatic – super open for better ones)