fix: line length in .flake8

[sbillinge]

Problem

We are using diffpy.utils as the example package in scikit-package but the .flake8 file still hsa our old standard of 115 character line length in it and not the new standard of 79. So running flake8 vs running pre-commit ressults in different outcomes (pre-commit seems to use the flake8 hook in .pre-commit-config to override whatever is in .flake8).

Proposed solution

change line-length in .flake8 to 79 and push out at least a minor release bump before scikit-package goes public.