Use proper gettext plural forms in optparse (closes #4391).

Original patch by Dwayne Bailey.
diff --git a/Lib/test/test_optparse.py b/Lib/test/test_optparse.py
index 7b95612..61f44ee 100644
--- a/Lib/test/test_optparse.py
+++ b/Lib/test/test_optparse.py
@@ -631,7 +631,7 @@
                                                option_list=options)
 
     def test_required_value(self):
-        self.assertParseFail(["-a"], "-a option requires an argument")
+        self.assertParseFail(["-a"], "-a option requires 1 argument")
 
     def test_invalid_integer(self):
         self.assertParseFail(["-b", "5x"],