[3.13] gh-130177: Fix unintentionally skipped tests in test_gettext (GH-130178) (GH-130183)

(cherry picked from commit 2e8044a)

Co-authored-by: Tomas R <[email protected]>

Author: miss-islington

Lib/test/test_gettext.py

@@ -452,7 +452,7 @@ def test_plural_context_forms_null_translations(self):
             numbers_only=False)
 
 
-class PluralFormsInternalTestCase:
+class PluralFormsInternalTestCase(unittest.TestCase):
     # Examples from http://www.gnu.org/software/gettext/manual/gettext.html
 
     def test_ja(self):