Clean up the warnings filter use in test_set.

Lib/test/test_set.py

@@ -907,7 +907,8 @@ class TestBasicOpsBytes(TestBasicOps):
 
 class TestBasicOpsMixedStringBytes(TestBasicOps):
     def setUp(self):
-        self.warning_filters = warnings.filters[:]
+        self._warning_filters = support.check_warnings()
+        self._warning_filters.__enter__()
         warnings.simplefilter('ignore', BytesWarning)
         self.case   = "string and bytes set"
         self.values = ["a", "b", b"a", b"b"]
@@ -917,7 +918,7 @@ class TestBasicOpsMixedStringBytes(TestBasicOps):
         self.repr   = "{'a', b'a', 'b', b'b'}"
 
     def tearDown(self):
-        warnings.filters = self.warning_filters
+        self._warning_filters.__exit__(None, None, None)
 
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