Add a test

The test proves that the `source and` clause is needed in
Distribution._read_egg_info_reqs().

Also, add a comment about the purpose of the third clause in the
Distribution.metadata() method.

Author: warsaw

importlib_metadata/api.py

@@ -198,6 +198,9 @@ def metadata(self):
         text = (
             self.read_text('METADATA')
             or self.read_text('PKG-INFO')
+            # This last clause is here to support old egg-info files.  Its
+            # effect is to just end up using the PathDistribution's self._path
+            # (which points to the egg-info file) attribute unchanged.
             or self.read_text('')
             )
         return _email_message_from_string(text)

importlib_metadata/tests/test_api.py

@@ -109,6 +109,10 @@ def test_version_egg_info_file(self):
         version = importlib_metadata.version('egginfo-file')
         self.assertEqual(version, '0.1')
 
+    def test_requires_egg_info_file(self):
+        requirements = importlib_metadata.requires('egginfo-file')
+        self.assertIsNone(requirements)
+
     def test_requires(self):
         deps = importlib_metadata.requires('egginfo-pkg')
         assert any(