Add AttrsInstance protocol to forgotten attrs.asdict and attrs.astuple

[tsx]

Summary

Fixes #1089

AttrsInstance protocol was added in #890 but two functions in the attrs namespace were apparently forgotten and still accept Any as the instance. This PR changes the signatures to accept AttrsInstance instead to match what's in the attr namespace.

Pull Request Check List

• Added tests for changed code.
  Our CI fails if coverage is not 100%.
• 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.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• 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.
• Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.

[hynek]

@tsx ah sorry can you add a news fragment describing the change in one sentence? I'm traveling rn otherwise I would do it myself. Thanks!

[tsx]

@hynek done. I tried to make it short and to the point.

[hynek]

Thanks! I've added a clarification that it's just about stubs, so people don't think there's a change in behavior.