Introduce importlib.abc. The module contains various ABCs related to imports

(mostly stuff specified by PEP 302). There are two ABCs, PyLoader and
PyPycLoader, which help with implementing source and source/bytecode loaders by
implementing load_module in terms of other methods. This removes a lot of
gritty details loaders typically have to worry about.

Lib/importlib/test/test_abc.py

@@ -0,0 +1,31 @@
+from importlib import abc
+from importlib import machinery
+import unittest
+
+
+class SubclassTests(unittest.TestCase):
+
+    """Test that the various classes in importlib are subclasses of the
+    expected ABCS."""
+
+    def verify(self, ABC, *classes):
+        """Verify the classes are subclasses of the ABC."""
+        for cls in classes:
+            self.assert_(issubclass(cls, ABC))
+
+    def test_Finder(self):
+        self.verify(abc.Finder, machinery.BuiltinImporter,
+                    machinery.FrozenImporter, machinery.PathFinder)
+
+    def test_Loader(self):
+        self.verify(abc.Loader, machinery.BuiltinImporter,
+                    machinery.FrozenImporter)
+
+
+def test_main():
+    from test.support import run_unittest
+    run_unittest(SubclassTests)
+
+
+if __name__ == '__main__':
+    test_main()