Remove python 3.7 tests
Remove python 3.7 tests

Currently gulp-flake8 has compatibility issues with 3.7.

petervanderdoes/wger,rolandgeider/wger,wger-project/wger,wger-project/wger,petervanderdoes/wger,petervanderdoes/wger,wger-project/wger,rolandgeider/wger,petervanderdoes/wger,wger-project/wger,rolandgeider/wger,rolandgeider/wger