Fix Python < 3.9 support

Author: ogrisel

Diff for: cloudpickle/cloudpickle.py

@@ -250,7 +250,7 @@ def _extract_code_globals(co):
         if co.co_consts:
             for const in co.co_consts:
                 if isinstance(const, types.CodeType):
-                    out_names |= _extract_code_globals(const)
+                    out_names.update(_extract_code_globals(const))
 
         _extract_code_globals_cache[co] = out_names
 

Diff for: tests/cloudpickle_test.py

@@ -2332,6 +2332,10 @@ def __type__(self):
         o = MyClass()
         pickle_depickle(o, protocol=self.protocol)
 
+    @pytest.mark.skipif(
+        sys.version_info < (3, 7),
+        reason="Determinism can only be guaranteed for Python 3.7+"
+    )
     def test_deterministic_pickle_bytes_for_function(self):
         # Ensure that functions with references to several global names are
         # pickled to fixed bytes that do not depend on the PYTHONHASHSEED of