flake8: increase the line length limit

The Linux kernel fairly recently adjusted the limit in checkpatch from 80 to 100 columns, I think it makes sense to follow suite. There are many places where the 80 column limit imposes on readability rather than improving it, 100 is a more sensible value.

It would be great to have some equivalent to # shellcheck ignore for times when exceeding the line limit makes sense and is agreed upon.