Don't pass maintainer_can_modify when patching the PR.

[Mariatta]

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

[codecov]

Codecov Report

Merging #163 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #163   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          20     20           
  Lines        1529   1529           
  Branches       86     86           
=====================================
  Hits         1529   1529

Impacted Files	Coverage Δ
bedevere/bpo.py	100% <100%> (ø)	⬆️
bedevere/close_pr.py	100% <100%> (ø)	⬆️

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[Mariatta]

I've deployed, and went through all the failed webhook deliveries fro the past 2 days. Things seem to be looking better now: PR's edited with link to bpo, bedevere statuses are being posted, and it still seems like I'm still able to edit the affected PRs.