flutter doctor fails: ios broken due python dependency

I did a brew upgrade and it upgraded to the latest python and broke Flutter.

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel beta, v0.1.5, on Mac OS X 10.13.3 17D102, locale en-DE)
[✓] Android toolchain - develop for Android devices (Android SDK 27.0.3)
[!] iOS toolchain - develop for iOS devices (Xcode 9.2)
✗ Missing Xcode dependency: Python module "six".
Install via 'pip install six' or 'sudo easy_install six'.
[✓] Android Studio (version 3.0)
[✓] VS Code (version 1.20.1)
[!] Connected devices
! No devices available

see:
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/ios/ios_workflow.dart#L41