Use proper lock object

Author: MikhailArkhipov

src/Analysis/Engine/Impl/Interpreter/Ast/AstPythonType.cs

@@ -246,7 +246,7 @@ public IEnumerable<string> GetMemberNames(IModuleContext moduleContext) {
         }
 
         protected bool ContainsMember(string name) {
-            lock (_members) {
+            lock (_lock) {
                 return Members.ContainsKey(name);
             }
         }