Fix message passing in frozen errors

Follow common exception workflows instead of working around it.

fixes #1514

Author: hynek

src/attr/exceptions.py

@@ -2,8 +2,6 @@
 
 from __future__ import annotations
 
-from typing import ClassVar
-
 
 class FrozenError(AttributeError):
     """
@@ -16,8 +14,10 @@ class FrozenError(AttributeError):
     .. versionadded:: 20.1.0
     """
 
-    msg = "can't set attribute"
-    args: ClassVar[tuple[str]] = [msg]
+    def __init__(self):
+        msg = "can't set attribute"
+        super().__init__(msg)
+        self.msg = msg
 
 
 class FrozenInstanceError(FrozenError):

tests/test_functional.py

@@ -264,13 +264,20 @@ def test_frozen_instance(self, frozen_class):
         """
         frozen = frozen_class(1)
 
-        with pytest.raises(FrozenInstanceError) as e:
+        with pytest.raises(
+            FrozenInstanceError, match="can't set attribute"
+        ) as e:
             frozen.x = 2
 
-        with pytest.raises(FrozenInstanceError) as e:
+        assert e.value.msg == e.value.args[0] == "can't set attribute"
+        assert 1 == frozen.x
+
+        with pytest.raises(
+            FrozenInstanceError, match="can't set attribute"
+        ) as e:
             del frozen.x
 
-        assert e.value.args[0] == "can't set attribute"
+        assert e.value.msg == e.value.args[0] == "can't set attribute"
         assert 1 == frozen.x
 
     @pytest.mark.parametrize(