Bump flake8 from 3.6.0 to 3.7.1
Bump flake8 from 3.6.0 to 3.7.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.6.0 to 3.7.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.6.0...3.7.1)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
marshmallow-code/apispec,marshmallow-code/smore