I'm not entirely sure, but I think it's probably a hard dependency on the homebrew version.  The python variables used below should be version-agnostic; You could try appending `=> :optional` or `=> :recommended` to the end of this line, and I think that should make it continue to use the system python.  I'd be interested to know if that works correctly; optional should prefer the system version, recommended should prefer the homebrew version (but both will allow either).