Issue #15902: Fix imp.load_module() to accept None as a file when

trying to load an extension module.

While at it, also add a proper unittest.skipIf() guard to another test
involving imp.load_dynamic().

Lib/test/test_imp.py

@@ -208,6 +208,8 @@ class ImportTests(unittest.TestCase):
             self.assertIsNot(orig_getenv, new_os.getenv)
 
     @support.cpython_only
+    @unittest.skipIf(not hasattr(imp, 'load_dynamic'),
+                     'imp.load_dynamic() required')
     def test_issue15828_load_extensions(self):
         # Issue 15828 picked up that the adapter between the old imp API
         # and importlib couldn't handle C extensions
@@ -230,6 +232,19 @@ class ImportTests(unittest.TestCase):
         self.assertIn(path, err.exception.path)
         self.assertEqual(name, err.exception.name)
 
+    @support.cpython_only
+    @unittest.skipIf(not hasattr(imp, 'load_dynamic'),
+                     'imp.load_dynamic() required')
+    def test_load_module_extension_file_is_None(self):
+        # When loading an extension module and the file is None, open one
+        # on the behalf of imp.load_dynamic().
+        # Issue #15902
+        name = '_heapq'
+        found = imp.find_module(name)
+        assert found[2][2] == imp.C_EXTENSION
+        found[0].close()
+        imp.load_module(name, None, *found[1:])
+
 
 class ReloadTests(unittest.TestCase):