Add xfail test capturing desired expectation.

Author: jaraco

tests/test_main.py

@@ -43,6 +43,11 @@ def test_package_not_found_mentions_metadata(self):
 
         assert "metadata" in str(ctx.exception)
 
+    def test_abc_enforced(self):
+        with self.assertRaises(AssertionError):  # xfail
+            with self.assertRaises(TypeError):
+                type('DistributionSubclass', (Distribution,), {})()
+
     @fixtures.parameterize(
         dict(name=None),
         dict(name=''),