Bump flake8 from 3.7.7 to 3.7.8 in /tests/ui/requirements (#8358)
Bump flake8 from 3.7.7 to 3.7.8 in /tests/ui/requirements (#8358)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.7 to 3.7.8.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.7...3.7.8)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
mozilla/addons-frontend,kumar303/addons-frontend,mozilla/addons-frontend,kumar303/addons-frontend,kumar303/addons-frontend,mozilla/addons-frontend,kumar303/addons-frontend,mozilla/addons-frontend