drop_duplicates() returns with error if column datatype is list
Can it be possible to improve `drop_duplicates()` method to handle `list` column datatpyes? Now if I have a column with `list` elements then `drop_duplicates()` throws the following exception:

> TypeError: type object argument after * must be a sequence, not map

Example:
```
df = pd.DataFrame({'a':[10,10,10,11,12], 'b':[['a', 'b'], ['a', 'b'], ['x', 'y'], ['a', 'b'], ['x', 'y']]})
df.drop_duplicates()
```
Now I have to create a new column that contains the original `list` elements as strings and I have to use this new column at  `drop_duplicates()`.

Thank you!
