Implement the PEP 302 protocol for get_filename() as

importlib.abc.ExecutionLoader. PyLoader now inherits from this ABC instead of
InspectLoader directly. Both PyLoader and PyPycLoader provide concrete
implementations of get_filename in terms of source_path and bytecode_path.

Lib/importlib/test/test_abc.py

@@ -53,9 +53,15 @@ class InspectLoader(InheritanceTests, unittest.TestCase):
                     machinery.FrozenImporter]
 
 
+class ExecutionLoader(InheritanceTests, unittest.TestCase):
+
+    superclasses = [abc.InspectLoader]
+    subclasses = [abc.PyLoader]
+
+
 class PyLoader(InheritanceTests, unittest.TestCase):
 
-    superclasses = [abc.Loader, abc.ResourceLoader, abc.InspectLoader]
+    superclasses = [abc.Loader, abc.ResourceLoader, abc.ExecutionLoader]
 
 
 class PyPycLoader(InheritanceTests, unittest.TestCase):