Update backoff to 1.7.0

[pyup-bot]

This PR updates backoff from 1.4.3 to 1.7.0.

Changelog

1.7.0

Changed

- Support Python 3.7
- Drop support for async in Python 3.4
- Drop support for Python 2.6
- Update development dependencies
- Use poetry for dependencies and packaging

1.6.0

Changed

- Change default log level from ERROR to INFO
- Log retries on exception as INFO

1.5.0

Changed

- Add max_time keyword argument

Links

• PyPI: https://pypi.org/project/backoff
• Changelog: https://pyup.io/changelogs/backoff/
• Repo: https://github.com/litl/backoff

[pyup-bot]

Closing this in favor of #453