Issue #15210: Greatly simplify the test for supporting importlib

working without _frozen_importlib by moving to an import over a direct
access in sys.modules.

Lib/importlib/__init__.py

@@ -6,8 +6,8 @@ import imp
 import sys
 
 try:
-    _bootstrap = sys.modules['_frozen_importlib']
-except KeyError:
+    import _frozen_importlib as _bootstrap
+except ImportError:
     from . import _bootstrap
     _bootstrap._setup(sys, imp)
 else:

Lib/importlib/test/test_api.py

 from . import util
 import imp
 import importlib
+from importlib import machinery
 import sys
+from test import support
 import unittest
 
 
@@ -164,18 +166,9 @@ class FrozenImportlibTests(unittest.TestCase):
 
     def test_no_frozen_importlib(self):
         # Should be able to import w/o _frozen_importlib being defined.
-        modules = {}
-        for name in ('importlib', 'importlib.__init__', 'importlib._bootstrap',
-                     '_frozen_importlib'):
-            try:
-                modules[name] = sys.modules[name]
-                del sys.modules[name]
-            except KeyError:
-                continue
-        modules['_frozen_importlib'] = None
-        import importlib
-        for name, module in modules.items():
-            sys.modules[name] = module
+        module = support.import_fresh_module('importlib', blocked=['_frozen_importlib'])
+        self.assertFalse(isinstance(module.__loader__,
+                                    machinery.FrozenImporter))
 
 
 def test_main():