Add ABCMeta to Distribution. Fixes #419.

Author: jaraco

Diff for: importlib_metadata/__init__.py

@@ -346,7 +346,7 @@ def __repr__(self):
         return f'<FileHash mode: {self.mode} value: {self.value}>'
 
 
-class Distribution:
+class Distribution(metaclass=abc.ABCMeta):
     """A Python distribution package."""
 
     @abc.abstractmethod

Diff for: tests/test_main.py

@@ -44,9 +44,8 @@ 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,), {})()
+        with self.assertRaises(TypeError):
+            type('DistributionSubclass', (Distribution,), {})()
 
     @fixtures.parameterize(
         dict(name=None),