Investigate Renovate for automated dependency updates

One-sentence description

Investigate Renovate for automated dependency updates

Motivation

Updates of our dependencies are rare (except for Django releases). This means our dependencies may be long out-of-date when we finally update them.

Desired functionality

Automated dependency updates.

Suggested implementation

Use https://github.com/renovatebot/renovate. However, it doesn't currently have support for Poetry. So we should wait for https://github.com/renovatebot/renovate/pull/3440