Match CPython wording for __slots__ conflict and __doc__ delete errors (#7698)

The behavior already matched CPython (the slot conflict is detected,
the __doc__ delete is rejected); only the message text drifted.

- "__slots__ conflicts with a class variable" -> drop the stray "a"
  to match CPython's "conflicts with class variable".
- "cannot delete '__doc__' attribute of type 'X'" -> insert "immutable"
  before "type" to match CPython's wording (CPython surfaces the same
  phrase even for user-defined classes since the descriptor refuses
  the delete unconditionally).

Lib/test/test_descr.py

@@ -4952,7 +4952,6 @@ class ClassPropertiesAndMethods(unittest.TestCase):
         with self.assertRaises(TypeError):
             a + a
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_slot_shadows_class_variable(self):
         with self.assertRaises(ValueError) as cm:
             class X:
@@ -4961,7 +4960,6 @@ class ClassPropertiesAndMethods(unittest.TestCase):
         m = str(cm.exception)
         self.assertEqual("'foo' in __slots__ conflicts with class variable", m)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_set_doc(self):
         class X:
             "elephant"

crates/vm/src/builtins/type.rs

@@ -2004,7 +2004,7 @@ impl Constructor for PyType {
                 // Check if slot name conflicts with class attributes
                 if attributes.contains_key(vm.ctx.intern_str(slot.as_wtf8())) {
                     return Err(vm.new_value_error(format!(
-                        "'{}' in __slots__ conflicts with a class variable",
+                        "'{}' in __slots__ conflicts with class variable",
                         slot.as_wtf8()
                     )));
                 }
@@ -2404,7 +2404,7 @@ impl Py<PyType> {
         // Similar to CPython's type_set_doc
         let value = value.ok_or_else(|| {
             vm.new_type_error(format!(
-                "cannot delete '__doc__' attribute of type '{}'",
+                "cannot delete '__doc__' attribute of immutable type '{}'",
                 self.name()
             ))
         })?;