Set ImportError attributes using _instance_setattr

Subclasses in the wild write `@property` functions that shadow the
attributes. Since they are getter only, the builtin attribute setting
fails with an `AttributeError`:

```
======================================================================
ERROR: test_set_path_attr_on_import_error_subclass_with_property_sets_attr (__main__.ExceptionTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "library/builtins_test.py", line 4968, in test_set_path_attr_on_import_error_subclass_with_property_sets_attr
    c = C("a path")
  File "library/builtins.py", line 1037, in __init__
    self.path = path
AttributeError: can't set attribute
```

If we use `_instance_setattr`, this works.

This may be a more general problem since CPython does all this stuff in
C and our builtins are all in Python, but I am not sure how to approach
that yet.

library/builtins.py

@@ -1031,8 +1031,8 @@ def __init__(self, *args, name=None, path=None):
         self.args = args
         if len(args) == 1:
             self.msg = args[0]
-        self.name = name
-        self.path = path
+        _instance_setattr(self, "name", name)
+        _instance_setattr(self, "path", path)
 
     def __reduce__(self):
         _unimplemented()

library/builtins_test.py

@@ -4944,6 +4944,32 @@ def test_system_exit_with_value_sets_code(self):
         exc = SystemExit(1111)
         self.assertEqual(exc.code, 1111)
 
+    def test_set_name_attr_on_import_error_subclass_with_property_sets_attr(self):
+        class C(ModuleNotFoundError):
+            # Modeled after PackageNotFoundError in
+            # importlib_metadata/__init__.py
+            @property
+            def name(self):
+                (name,) = self.args
+                return name
+
+        c = C("a name")
+        self.assertEqual(c.name, "a name")
+        self.assertEqual(c.path, None)
+
+    def test_set_path_attr_on_import_error_subclass_with_property_sets_attr(self):
+        class C(ModuleNotFoundError):
+            # Modeled after PackageNotFoundError in
+            # importlib_metadata/__init__.py
+            @property
+            def path(self):
+                (path,) = self.args
+                return path
+
+        c = C("a path")
+        self.assertEqual(c.name, None)
+        self.assertEqual(c.path, "a path")
+
 
 class EvalTests(unittest.TestCase):
     def test_globals_none_accesses_function_globals(self):