Add unsafe_hash alias for class-wide hash

[hynek]

Summary

attr.s(unsafe_hash=True) is now equivalent to attr.s(hash=True) to standardize based on PEP 681.

Fixes #1003

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]

@debonte does this cover what you asked me about in checks notes April? 🙈

[debonte]

@debonte does this cover what you asked me about in checks notes April? 🙈

:) I added one minor comment about the documentation. Otherwise, this looks good to me. Thanks for making this change!