In Sierra: How to get Qt 5.8.0 / PyQt 5.7.1 / sip 4.19 / Python3.5 to install & work correctly?
I am the one who did the Qt5 5.8.0 upgrade for Homebrew. This is a known problem that will persist until Riverbank upgrades their software. I suggest you contact them on their mailing list.

UPDATE: As a temporary workaround, I've added a qt@5.7 formula, and updated the PyQt5 formula to depend on it. See the following pull requests, which I've merged:

https://github.com/Homebrew/homebrew-core/pull/9297

https://github.com/Homebrew/homebrew-core/pull/9308