python,删除列表中的所有素数

a = [3, 4, 7, 4, 3, 5, 6, 9]
b=[]
for s in a:
       for i in range(2, s):
           if(s % i==0):
               break;
           if(i==s-1):
              b.append(s)

for ss in b:
    while ss in a:
        a.remove(ss)
print(a)