SubsetTransformer is missing alias after deepcopy

[micahjsmith]

• ballet version: 0.19.3

Description

Mysteriously, the subset transformer is missing the attribute alias after a deepcopy operation

What I Did

from ballet.eng import SubsetTransformer, IdentityTransformer
from copy import deepcopy
t = SubsetTransformer('a', IdentityTransformer())
t.alias  # okay
deepcopy(t).alias  # error

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-42-82dcfda2edba> in <module>
----> 1 deepcopy(t).alias

AttributeError: 'SubsetTransformer' object has no attribute 'alias'

[micahjsmith]

On sklearn-pandas 1.x, __setstate__ (but not __getstate__!) is implemented to provide compatibility with older versions of itself. This implementation does not call super so classes that inherit from DataFrameMapper (as SubsetTransformer does) lose their new attributes.