Do not copy __weakref__ from original _cls_dict (#410)

self._cls_dict["__weakref__"] holds a reference to self._cls, preventing
self._cls from being released after the new, slots-enabled class is
returned.

Fixes #407

changelog.d/407.change.rst

@@ -0,0 +1 @@
+Fixed a reference leak where the original class would remain live after being replaced when ``slots=True`` is set.

src/attr/_make.py

@@ -493,7 +493,7 @@ class _ClassBuilder(object):
         cd = {
             k: v
             for k, v in iteritems(self._cls_dict)
-            if k not in tuple(self._attr_names) + ("__dict__",)
+            if k not in tuple(self._attr_names) + ("__dict__", "__weakref__")
         }
 
         # We only add the names of attributes that aren't inherited.

tests/test_make.py

@@ -5,6 +5,7 @@ Tests for `attr._make`.
 from __future__ import absolute_import, division, print_function
 
 import copy
+import gc
 import inspect
 import itertools
 import sys
@@ -1250,6 +1251,25 @@ class TestClassBuilder(object):
 
         assert C() == copy.deepcopy(C())
 
+    def test_no_references_to_original(self):
+        """
+        When subclassing a slots class, there are no stray references to the
+        original class.
+        """
+
+        @attr.s(slots=True)
+        class C(object):
+            pass
+
+        @attr.s(slots=True)
+        class C2(C):
+            pass
+
+        # The original C2 is in a reference cycle, so force a collect:
+        gc.collect()
+
+        assert [C2] == C.__subclasses__()
+
 
 class TestMakeCmp:
     """