gh-124176: Add special support for dataclasses to create_autospec

[sobolevn]

Now creating autospecs from dataclass types is easier. We inspect all fields and add them to __dir__, so mock's spec will know about future dataclass instance fields.

• Issue: Improve UX for mocking dataclasses that are problematic to instantiate #124176

[ericvsmith]

I'm not a user of unittest.mock, but these changes look reasonable to me.

[carljm]

This looks reasonable to me as well.

[cjw296]

Supportive of the change, just the code hygiene changes to make.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[sobolevn]

@cjw296 done! 👍

[ncoghlan]

LGTM! I especially like the test coverage.

[cjw296]

Nice!

[sobolevn]

Thanks everyone! 🎉