Not sure where you get this from. The most popular machine learning framework, scikit-learn, runs fine in 3.5. I am also using xgboost with Python 3.5 (yes, xgboost is a major open source for machine learning, just look at what framework is used by most Kaggle competition winners). TensorFlow, mxnet also support 3.5. I would have agreed with you a year ago, but there has been a major shift in use from 2.7 to 3.5 in 2016.

edited for typo.