Issue #13703: add a way to randomize the hash values of basic types (str, bytes, datetime)

in order to make algorithmic complexity attacks on (e.g.) web apps much more complicated.

The environment variable PYTHONHASHSEED and the new command line flag -R control this
behavior.

Lib/test/mapping_tests.py

@@ -14,7 +14,7 @@ class BasicTestMappingProtocol(unittest.TestCase):
     def _reference(self):
         """Return a dictionary of values which are invariant by storage
         in the object under test."""
-        return {1:2, "key1":"value1", "key2":(1,2,3)}
+        return {"1": "2", "key1":"value1", "key2":(1,2,3)}
     def _empty_mapping(self):
         """Return an empty mapping object"""
         return self.type2test()