История изменений

https://docs.python.org/3/library/functions.html#exec

exec(object[, globals[, locals]])

If the globals dictionary does not contain a value for the key __builtins__, a reference to the dictionary of the built-in module builtins is inserted under that key. That way you can control what builtins are available to the executed code by inserting your own __builtins__ dictionary into globals before passing it to exec().

к вопросу об импорте левых модулей:

>>> exec("import os")
>>> exec("import os", {'__builtins__': {}})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
ImportError: __import__ not found

https://docs.python.org/3/library/functions.html#exec

exec(object[, globals[, locals]])

If the globals dictionary does not contain a value for the key __builtins__, a reference to the dictionary of the built-in module builtins is inserted under that key. That way you can control what builtins are available to the executed code by inserting your own __builtins__ dictionary into globals before passing it to exec().