Fix @Frozen exceptions to allow __traceback__ to be set.

E.g. contextlib.contextmanager does so whenever an exception is
raised in its body, and does so even on CPython, so merging the
two code paths now seems reasonable.

src/attr/_make.py

@@ -12,7 +12,7 @@
 # We need to import _compat itself in addition to the _compat members to avoid
 # having the thread-local in the globals here.
 from . import _compat, _config, setters
-from ._compat import PY310, PYPY, _AnnotationExtractor, set_closure_cell
+from ._compat import PY310, _AnnotationExtractor, set_closure_cell
 from .exceptions import (
     DefaultAlreadySetError,
     FrozenInstanceError,
@@ -582,28 +582,19 @@ def _transform_attrs(
     return _Attributes((AttrsClass(attrs), base_attrs, base_attr_map))
 
 
-if PYPY:
-
-    def _frozen_setattrs(self, name, value):
-        """
-        Attached to frozen classes as __setattr__.
-        """
-        if isinstance(self, BaseException) and name in (
-            "__cause__",
-            "__context__",
-        ):
-            BaseException.__setattr__(self, name, value)
-            return
-
-        raise FrozenInstanceError()
-
-else:
+def _frozen_setattrs(self, name, value):
+    """
+    Attached to frozen classes as __setattr__.
+    """
+    if isinstance(self, BaseException) and name in (
+        "__cause__",
+        "__context__",
+        "__traceback__",
+    ):
+        BaseException.__setattr__(self, name, value)
+        return
 
-    def _frozen_setattrs(self, name, value):
-        """
-        Attached to frozen classes as __setattr__.
-        """
-        raise FrozenInstanceError()
+    raise FrozenInstanceError()
 
 
 def _frozen_delattrs(self, name):

tests/test_next_gen.py

@@ -6,6 +6,7 @@
 
 import re
 
+from contextlib import contextmanager
 from functools import partial
 
 import pytest
@@ -312,6 +313,38 @@ class MyException(Exception):
         assert "foo" == ei.value.x
         assert ei.value.__cause__ is None
 
+    @pytest.mark.parametrize(
+        "decorator",
+        [
+            partial(_attr.s, frozen=True, slots=True, auto_exc=True),
+            attrs.frozen,
+            attrs.define,
+            attrs.mutable,
+        ],
+    )
+    def test_setting_traceback_on_exception(self, decorator):
+        """
+        contextlib.contextlib (re-)sets __traceback__ on raised exceptions.
+
+        Ensure that works, as well as if done explicitly
+        """
+
+        @decorator
+        class MyException(Exception):
+            pass
+
+        @contextmanager
+        def do_nothing():
+            yield
+
+        with do_nothing(), pytest.raises(MyException) as ei:
+            raise MyException()
+
+        assert isinstance(ei.value, MyException)
+
+        # this should not raise an exception either
+        ei.value.__traceback__ = ei.value.__traceback__
+
     def test_converts_and_validates_by_default(self):
         """
         If no on_setattr is set, assume setters.convert, setters.validate.