Add a test for a single-character apostrophe string

We recently fixed a bug where a single-character string consisting solely
of the apostrophe character raised an exception. Add this test case to the
test suite.

tests/test_pwd.py

@@ -743,6 +743,7 @@ def test__pl_general_verb(self):
             ("tuna's", "tuna's"),
             ("TUNA's", "TUNA's"),
             ("bad", False),
+            ("'", False),
             pytest.param(
                 "JOHN's",
                 "JOHNS'",