fixed Python derived types crashing on shutdown of Python process

clearing GCHandle from an instance of Python derived type would drop the last reference to it, so it was destroyed without being removed from reflectedObjects collection

src/runtime/extensiontype.cs

@@ -80,16 +80,17 @@ namespace Python.Runtime
 
             tp_clear(lastRef.Borrow());
 
-            bool deleted = loadedExtensions.Remove(lastRef.DangerousGetAddress());
-            Debug.Assert(deleted);
-
             // we must decref our type: https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_dealloc
             DecrefTypeAndFree(lastRef.Steal());
         }
 
         public static int tp_clear(BorrowedReference ob)
         {
-            TryFreeGCHandle(ob);
+            if (TryFreeGCHandle(ob))
+            {
+                bool deleted = loadedExtensions.Remove(ob.DangerousGetAddress());
+                Debug.Assert(deleted);
+            }
 
             int res = ClassBase.BaseUnmanagedClear(ob);
             return res;

src/runtime/runtime.cs

@@ -339,6 +339,7 @@ namespace Python.Runtime
                 else if (forceBreakLoops)
                 {
                     NullGCHandles(CLRObject.reflectedObjects);
+                    CLRObject.reflectedObjects.Clear();
                 }
             }
             return false;