Add Placeholder to __all__

Author: rhettinger

Lib/functools.py

@@ -12,7 +12,7 @@
 __all__ = ['update_wrapper', 'wraps', 'WRAPPER_ASSIGNMENTS', 'WRAPPER_UPDATES',
            'total_ordering', 'cache', 'cmp_to_key', 'lru_cache', 'reduce',
            'partial', 'partialmethod', 'singledispatch', 'singledispatchmethod',
-           'cached_property']
+           'cached_property', 'Placeholder']
 
 from abc import get_cache_token
 from collections import namedtuple