Optimize build script

[rllola]

You don't need to build everything for pyqt5 and avoid redundant hardcoded version

See https://github.com/rllola/WebBrowser-PyQt5/blob/master/get-pyqt5-osx.sh

• [ ] Don't build all the modules
• [ ] Use variable for version in script
• [ ] Add checksum verification

[rllola]

Irrelevant anymore because we have migrated to python3 and are using pip to install pyqt5