define __slots__ in all base classes

...otherwise there it does not do anything. add a test for this.

src/sanest/sanest.py

@@ -16,7 +16,7 @@ class Collection(
             collections.abc.Sized,
             collections.abc.Iterable,
             collections.abc.Container):
-        pass
+        __slots__ = ()
 
 ATOMIC_TYPES = (bool, float, int, str)
 CONTAINER_TYPES = (builtins.dict, builtins.list)
@@ -404,6 +404,8 @@ class SaneCollection(Collection):
     """
     Base class for ``sanest.dict`` and ``sanest.list``.
     """
+    __slots__ = ()
+
     @abc.abstractmethod
     def wrap(cls, data, *, check=True):
         raise NotImplementedError  # pragma: no cover

test_sanest.py

@@ -1669,6 +1669,15 @@ def test_missing_arg_repr():
     assert str(_sanest.MISSING) == '<missing>'
 
 
+def test_slots():
+    d = sanest.dict()
+    with pytest.raises(AttributeError):
+        d.foo = 123
+    l = sanest.list()
+    with pytest.raises(AttributeError):
+        l.foo = 123
+
+
 def dedent(s):
     s = s.lstrip('\n')
     s = textwrap.dedent(s).rstrip()