bpo-41338: Fix DeprecationWarning in tests (GH-21542)

diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 9dbae44..5d0992a 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -38,6 +38,7 @@
 from test.support import (TestFailed,
                           run_with_locale, cpython_only)
 from test.support.import_helper import import_fresh_module
+from test.support import warnings_helper
 import random
 import inspect
 import threading
@@ -584,6 +585,7 @@
 
     @cpython_only
     @requires_legacy_unicode_capi
+    @warnings_helper.ignore_warnings(category=DeprecationWarning)
     def test_from_legacy_strings(self):
         import _testcapi
         Decimal = self.decimal.Decimal
@@ -2820,6 +2822,7 @@
 
     @cpython_only
     @requires_legacy_unicode_capi
+    @warnings_helper.ignore_warnings(category=DeprecationWarning)
     def test_from_legacy_strings(self):
         import _testcapi
         c = self.decimal.Context()