bpo-33509: Fix test_warnings for python3 -Werror (GH-7365)

Fix test_warnings.test_module_globals() when python3 is run with
-Werror.
(cherry picked from commit e292b75e3e)

Co-authored-by: Victor Stinner <vstinner@redhat.com>

Lib/test/test_warnings/__init__.py

@@ -221,6 +221,8 @@ class FilterTests(BaseTest):
     def test_module_globals(self):
         with original_warnings.catch_warnings(record=True,
                 module=self.module) as w:
+            self.module.simplefilter("always", UserWarning)
+
             # bpo-33509: module_globals=None must not crash
             self.module.warn_explicit('msg', UserWarning, "filename", 42,
                                       module_globals=None)