Qwt 6.1.0 upgrade is causing crashes or build problems
Downgrade to Qwt 6.0.1 and reinstall
$ brew remove qwt
$ git checkout 4392eba Library/Formula/qwt.rb
$ brew install qwt
QGIS shows SIP error messages on startup, indicating that QGIS is using the system Python instead of Homebrew's Python