Xcode 7.3.1 stack is never updated. That’s an LTS stack since Xcode 7.3.1 came out, there was absolutely no update on it since - for more information about stack and update types see:
You should check brew about this, I’d say most likely the python3 brew formula did change, and now depends on a different version of openssl. It definitely is not related to any update we did on the stack, as other than the Bitrise CLI updates there was absolutely no other update on the stack since Xcode 7.3.1 / May 2016 (http://blog.bitrise.io/2016/05/05/xcode-7-3-1-stack-update.html).