set applied to a string is not the set containing the string

[MalteKurz]

set('abc') = {'a', 'b', 'c'}, so we need to add brackets around self.y_col here:

doubleml-for-py/doubleml/double_ml_data.py

Line 259 in be32d1f

y_d = set.union(set(self.y_col), set(self.d_cols))

--> also at other places
--> related to #84