Python: Port ShouldBeContextManager.ql

Only trivial test changes.

Author: tausbn

python/ql/src/Classes/ShouldBeContextManager.ql

@@ -14,10 +14,12 @@
  */
 
 import python
-private import LegacyPointsTo
+private import semmle.python.dataflow.new.internal.DataFlowDispatch
 
-from ClassValue c
-where not c.isBuiltin() and not c.isContextManager() and exists(c.declaredAttribute("__del__"))
+from Class c
+where
+  not DuckTyping::isContextManager(c) and
+  DuckTyping::hasMethod(c, "__del__")
 select c,
   "Class " + c.getName() +
     " implements __del__ (presumably to release some resource). Consider making it a context manager."

python/ql/test/query-tests/Classes/should-be-context-manager/ShouldBeContextManager.expected

@@ -1,2 +1,2 @@
-| should_be_context_manager.py:3:1:3:22 | class MegaDel | Class MegaDel implements __del__ (presumably to release some resource). Consider making it a context manager. |
-| should_be_context_manager.py:16:1:16:22 | class MiniDel | Class MiniDel implements __del__ (presumably to release some resource). Consider making it a context manager. |
+| should_be_context_manager.py:3:1:3:22 | Class MegaDel | Class MegaDel implements __del__ (presumably to release some resource). Consider making it a context manager. |
+| should_be_context_manager.py:16:1:16:22 | Class MiniDel | Class MiniDel implements __del__ (presumably to release some resource). Consider making it a context manager. |