Issue #23075: Whether __builtins__ is a module or a dict is undefined in

CPython. Use the reliably well defined `import __builtin__` instead.
Patch by Alex Gaynor.

Lib/test/_mock_backport.py

@@ -24,6 +24,7 @@ __all__ = (
 __version__ = '1.0'
 
 
+import __builtin__
 import inspect
 import pprint
 import sys
@@ -32,7 +33,7 @@ from types import ModuleType
 from functools import wraps, partial
 
 
-_builtins = {name for name in __builtins__ if not name.startswith('_')}
+_builtins = {name for name in dir(__builtin__) if not name.startswith('_')}
 
 BaseExceptions = (BaseException,)
 if 'java' in sys.platform: