Show astype example with dict in docs
It would be useful to be able to change a column dtype inline.

For example with a DataFrame-level astype() overload:
Assuming `df` has a column called `str_col` that is a string (object),
`df.astype(col='str_col', dtype='int').head()`

The use case that brought me here is to numerically sort a string column without permanently changing it. What I was hoping to do is
`df.astype(col='str_col', dtype='int').sort_values(by='str_col')`

Instead, I had to do this:
`df = df.assign(int_col = lambda x: x['str_col'].astype('int')).sort_values(by='int_col').drop('int_col', axis = 'columns')`

The method would also support dictionaries for multiple column type changes:
`df.astype({'col1':'type1', 'col2':'type2'})`