annotate：聚合分组，对应sql语句的group by
第一步：导入
from django.db.models.aggregates import Count
第二步：应用
res = Students.objects.values(‘gender’).annotate(people=Count(‘gender’))
对应sql：
SELECT M3_students.gender, COUNT(M3_students.gender) AS people FROM M3_students GROUP BY M3_students.gender ORDER BY NULL