[BUG] Series doesn't support attribute access

[ericpai]

The Series doesn't support attribute access, but the pandas' can.

To Reproduce

import mars.dataframe as md
s1 = md.Series([1, 2, 3], index=list("abc"))
print(s1.b)

>>> AttributeError: 'SeriesData' object has no attribute 'b'

Expected behavior

>>> 2