We used to need to pass the maintainer_can_modify flag, but now we're getting error message 422 Validation Failed for 'maintainer_can_modify' when patching the PR. (to append the bpo link)

Removing the maintainer_can_modify flag fixes the problem, I tested in command line.
Hopefully this quick fix can unblock people.

Perhaps something changed from GitHub side, but I'm not seeing anything obvious in their changelog.

Closes #162
Closes python/core-workflow#321