Set `pr_number` attribute when PR is created for programmatic usage of cherry-picker

[Jackenmen]

Simply setting PR number under self.pr_number after successfully creating PR would be enough:

cherry-picker/cherry_picker/cherry_picker.py

Line 334 in 6b302ad

click.echo(f"Backport PR created at {response.json()['html_url']}")

In addition to allowing to refer to the PR after the backport, it also allows checking if the PR was actually created successfully which right now is not possible either (unless you catch stdout but that's quite bothersome in comparison to simple API modification in this project).