Update flake8 requirement from <3.9.0,>=3.7.9 to >=3.7.9,<3.10.0 (#636)
Update flake8 requirement from <3.9.0,>=3.7.9 to >=3.7.9,<3.10.0 (#636)

Updates the requirements on [flake8](https://gitlab.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.9...3.9.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
larq/larq