Bump flake8 from 3.7.8 to 3.7.9 in /requirements (#5559)
Bump flake8 from 3.7.8 to 3.7.9 in /requirements (#5559)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.8 to 3.7.9.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.8...3.7.9)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
KWierso/treeherder,KWierso/treeherder,KWierso/treeherder,KWierso/treeherder