Move a test-skipping decorator over to unittest.skipIf.

Lib/importlib/test/import_/util.py

 import functools
+import importlib
 import importlib._bootstrap
+import unittest
 
 
 using___import__ = False
@@ -13,16 +15,7 @@ def import_(*args, **kwargs):
         return importlib._bootstrap.__import__(*args, **kwargs)
 
 
-def importlib_only(fxn):
-    """Decorator to mark which tests are not supported by the current
-    implementation of __import__()."""
-    def inner(*args, **kwargs):
-        if using___import__:
-            return
-        else:
-            return fxn(*args, **kwargs)
-    functools.update_wrapper(inner, fxn)
-    return inner
+importlib_only = unittest.skipIf(using___import__, "importlib-specific test")
 
 
 def mock_path_hook(*entries, importer):