I've noticed that in 2.6.0 version(2019-02-06) cell.column no longer returns column's letter, but column's number. Is this bug?
It's not a bug but it is a change in the API to have more consistency in things like ranges and charts where numerical indexing is more common. column_letter was introduced as an alias a while ago and is what you should when you want the letter.
I guess this is a difficult one but within the library itself the numerical index is used far more often.