Correctly handle dependencies on issues

[Levi-Lesches]

I tried using Depends on: #ISSUE_NUMBER in my PR, and dPulls marked the PR as ready to merge, even though the issue was still open.

I need this workflow since sometimes PRs are created before their dependent PRs are. For example:

• There are two features, A and B, with open issues for both
• A depends on B
• Author B is busy and author A wants to get started
• Author A opens PR A and marks it dependent on issue B
• Author B opens PR B and links it to issue B
• Author B merges PR B and thus issue B is automatically closed
• PR A notes that issue B is closed and is marked as ready to merge

This way, PR A can be created and marked as dependent even before PR B is created.