Sorry, I got the levels slightly confused there - a bit more experimentation shows it isn't the parent package that needs a __spec__ attribute, but only the package being executed.

# Parent package needs __path__ rather than __spec__
>>> find_spec("unittest.mock")
ModuleSpec(name='unittest.mock', loader=<_frozen_importlib_external.SourceFileLoader object at 0x7fa7b6434b38>, origin='/usr/lib64/python3.5/unittest/mock.py')
>>> sys.modules["unittest"] = object()
>>> find_spec("unittest.mock")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/importlib/util.py", line 89, in find_spec
    return _find_spec(fullname, parent.__path__)
AttributeError: 'object' object has no attribute '__path__'

# Leaf package needs __spec__
>>> del sys.modules["unittest"]
>>> find_spec("unittest.mock")
ModuleSpec(name='unittest.mock', loader=<_frozen_importlib_external.SourceFileLoader object at 0x7fa7b64344e0>, origin='/usr/lib64/python3.5/unittest/mock.py')
>>> import unittest.mock
>>> del sys.modules["unittest.mock"].__spec__
>>> find_spec("unittest.mock")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/importlib/util.py", line 99, in find_spec
    raise ValueError('{}.__spec__ is not set'.format(name)) from None
ValueError: unittest.mock.__spec__ is not set