Came across the question myself today. I think the reason that your code returns 'None' (exactly what I got by using the same method) is that
a.sort()
is calling the sort function to mutate the list a. In my understanding, this is a modification command. To see the result you have to use print(a).
My solution, as I tried to keep everything in pandas:
pd.Series(df['A'].unique()).sort_values()