Update README.md

Fix #133. Thanks @zackw for the report!

Author: joerick

README.md

@@ -381,6 +381,7 @@ If your wheel didn't compile, check the list below for some debugging tips.
 - A mistake in your config. To quickly test your config without doing a git push and waiting for your code to build on CI, you can run the Linux build in a Docker container. On Mac or Linux, with Docker running, try `cibuildwheel --platform linux`. You'll have to bring your config into the current environment first.
 - Missing dependency. You might need to install something on the build machine. You can do this in `.travis.yml`, `appveyor.yml`, or `.circleci/config.yml`, with apt-get, brew or whatever Windows uses :P . Given how the Linux build works, we'll probably have to build something into `cibuildwheel`. Let's chat about that over in the issues!
 - Windows: missing C feature. The Windows C compiler doesn't support C language features invented after 1990, so you'll have to backport your C code to C90. For me, this mostly involved putting my variable declarations at the top of the function like an animal.
+- MacOS: calling cibuildwheel from a python3 script and getting a `ModuleNotFoundError`? Due to a [bug](https://bugs.python.org/issue22490) in CPython, you'll need to [unset the `__PYVENV_LAUNCHER__` variable](https://github.com/joerick/cibuildwheel/issues/133#issuecomment-478288597) before activating a venv.
 
 Working examples
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