Bump flake8 from 3.6.0 to 3.7.0
Bump flake8 from 3.6.0 to 3.7.0

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.6.0 to 3.7.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.6.0...3.7.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
maximkulkin/marshmallow-oneofschema