Unfortunately I don't know macs. Brew likely installed it in the wrong place or the wrong python version is in your path. Maybe google can help you finding what to copy where. Another way would be to use
https://www.continuum.io/downloads instead of pure python. Anaconda should be more easy to use and has its own package manager with many prebuild packages. Maybe it even ships with pyqt, otherwise
conda install pyqt
Will install it. To start comictagger with it, you need to make shure to start it with anacondas python. Maybe it automatically configeres your path, then just
python comictagger.py
Will suffice. Otherwise replace python with the path to anacondas python executable.