Create an Index with values cast to dtypes. The class of a new Index
is determined by dtype. When conversion is impossible, a ValueError
exception is raised.
Note that any signed integer dtype is treated as 'int64',
and any unsigned integer dtype is treated as 'uint64',
regardless of the size.
By default, astype always returns a newly allocated object.
If copy is set to False and internal requirements on dtype are
satisfied, the original data is used to create a new Index
or the original Index is returned.
Index with values cast to specified dtype.