fitwidth doesn't always get correct window width

[pfuntner]

I think I've noticed this in the past but I am just getting around to opening a story about it:

$ python -c 'print("*"*500)'
********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
$ python -c 'print("*"*500)' | fitwidth
************************************************************************************************************************************************************************************************************************************************************************** ...
$ python -c 'print("*"*500)' | fitwidth | head
*************************************************************************** ...
$ cols
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111222222222222222222222222222222222222222222222222222222222222222222222222
0000000001111111111222222222233333333334444444444555555555566666666667777777777888888888899999999990000000000111111111122222222223333333333444444444455555555556666666666777777777788888888889999999999000000000011111111112222222222333333333344444444445555555555666666666677
1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901
$

I think I changed things recently to use a different method of determining the width but when stdout of fitwidth isn't a terminal, the result can be wrong - it looks like it's falling back on a default of 80 characters. I might need to get the infomation specifically from /dev/tty instead of stdout... I don't even know if that's possible but this is a problem.