I've just encountered what appears to be a different manifestation of the same issue. I unlinked the .tox directory and that rendered importlib broken.

$ pwd
/Users/jaraco/yg/queso
$ rm -R .tox
$ python
Python 3.6.0rc1 (v3.6.0rc1:29a273eee9a5, Dec  6 2016, 16:24:13) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> ^D
$ python -m tox
Could not import runpy module
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 15, in <module>
    import importlib.util
ModuleNotFoundError: No module named 'importlib.util'
$ ls /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/importlib
__init__.py		__pycache__		_bootstrap.py		_bootstrap_external.py	abc.py			machinery.py

Although I'm not certain that another process wasn't involved, I'm confident that it was the removal of the .tox directory (and interaction with Dropbox) that caused the corruption.