Fix @frozen exceptions to allow __traceback__ to be set.

[Julian]

Summary

This code previously raised an attrs.FrozenInstanceError:

from contextlib import contextmanager
from attrs import frozen

@frozen
class Boom(Exception):
    pass


@contextmanager
def do_nothing():
    yield


with do_nothing() as _:
    raise Boom()

on CPython (3.11 specifically, but probably other versions). On PyPy, it works, at least on 3.9 here -- which means I think it seems reasonable to merge the two codepaths here as I've done rather than whitelisting different attributes on different interpreters, but lemme know if you disagree.

(In real life a likely way you're going to encounter this, just to help anyone searching, is that click will wrap all exceptions during execution of a program, and does so via a contextlib.contextmanager)

Refs #712

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]

Ah nice I'm pretty sure I ran junk this myself when switching to 3.11 but decided to just thaw the class and get on with my life.

Can you finish it up with a news fragment pls?

[Julian]

Fixed!

[hynek]

Sir this is a Wendy's Markdown shop. 🤓

[Julian]

Ups, fix fixed then.

[hynek]

Excellent thank you!