The drug being tested in this study is called pevonedistat. Pevonedistat is being tested to treat people with higher-risk myelodysplastic syndromes (MDS), chronic myelomonocytic leukemia (CMML) and low-blast acute myelogenous leukemia (AML) as a combination treatment with azacitidine. This study will look at the overall survival, event-free survival and response to treatment in people who take pevonedistat and azacitidine when compared to people who take single-agent azacitidine.