WIP: Add an auto-converter, converters and validators

[sscherfke]

Fixes: #649

Pull Request Check List

This is just a friendly reminder about the most common mistakes. Please make sure that you tick all boxes. But please read our contribution guide at least once, it will save you unnecessary review cycles!

If an item doesn't apply to your pull request, check it anyway to make it apparent that there's nothing left to do. If your pull request is a documentation fix or a trivial typo, feel free to delete the whole thing.

• Added tests for changed code.
• New features have been added to our Hypothesis testing strategy.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file tests/typing_example.py.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signature of @attr.s() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in our __init__.py file.
• Documentation in .rst files is written using semantic newlines.
• Changes (and possible deprecations) have news fragments in changelog.d.

If you have any questions to any of the points above, just submit and ask! This checklist is here to help you, not to deter you from contributing!

[sscherfke]

This is PR is based on #653 and adds actual converters and validators for attrs classes. These should be complete enough to cover most use cases for API clients and settings loaders.

The current state of the PR should be functionally complete, but some docs are still missing.

I will be available for help if bugs or issues should arrise with the code of this PR.

[sscherfke]

@hynek What do you generally think about this PR? Shall I continue working on it?

[hynek]

Hmmm I think this would be better served as an external proof of concept before we bake it into attrs?

[sscherfke]

I could put it into typed-settings for a while. From there I could also use it for some API clients.

Should this PR remain open?

[hynek]

We should have a place to regularly touch base but maybe your original ticket is better for that, since it has more history?

[sscherfke]

Okay, let's do that. I’ll create a new PR once you think these changes are ready to be integrated into attrs.