Astrea

Another rebase has been done to match the branch closely with 5.12.0. It's unlikely that I'll do any more rebases.

To clarify: it should be as simple as adding an extra field to the class to serve as a blacklist of members (and adding basic logic to filter out said...

Honestly, that's the idea I had (more or less). Though I'd also rather not have the `add_black/whitelist` to prevent people from running into unexpected behavior (lots of debate can be...

I'm... not actually sure if this is really useful, honestly. This seems like a niche usecase that isn't done terribly often, and it's only a few lines of code.

> Your code sounds cool, but out of interest how are you validating for objects that should _not_ be exposed to the top level API of a library? Its not...

For transparency's sake, since it's been some months and this PR would have to be recreated if we want to continue with it, here's the script that generated everything: https://gist.github.com/AstreaTSS/9032753c1ae1b8ece431a87d31ca166f

Typically, input errors like this are just handled through `ValueError`. I don't see a need of adding a new type of error.

I will note that minimum versions are less about stability (in my eyes) than they are adoption - too new of a version means we lose out on a number...