Python: Make isNewType more precise

For module-level metaclass declarations, we now also check that the
right hand side in a `__metaclass__ = type` assignment is in fact the
built-in `type`.

Author: tausbn

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowDispatch.qll

@@ -2108,7 +2108,8 @@ module DuckTyping {
       // Module-level __metaclass__ = type makes all classes in the module new-style
       exists(Assign a |
         a.getScope() = cls.getEnclosingModule() and
-        a.getATarget().(Name).getId() = "__metaclass__"
+        a.getATarget().(Name).getId() = "__metaclass__" and
+        a.getValue() = API::builtin("type").getAValueReachableFromSource().asExpr()
       )
     )
   }